Make ActionView::LookupContext::DetailsKey thread-safe

Hi, we recently upgraded to Rails 6 and have had some 'random' issues
pop up twice now in the last two weeks where upon scaling our
application instances we found a single instance failing when trying to
render with stack traces that looked a bit like this:

```
vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/base.rb:274:in `_run': undefined method `_app_views_dealers_call_centre_index_html_haml__1117437046933042982_70191578841420' for #<#<Class:0x00007fad784998c8>:0x00007fad6ae237a8>
Did you mean?  _app_views_dealers_active_leads_index_html_haml__2989992190908713101_70191398405520 (ActionView::Template::Error)
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/template.rb:185:in `block in render'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications.rb:180:in `block in instrument'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications/instrumenter.rb:24:in `instrument'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications.rb:180:in `instrument'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/template.rb:386:in `instrument_render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/template.rb:183:in `render'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/template_renderer.rb:59:in `block (2 levels) in render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/abstract_renderer.rb:89:in `block in instrument'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications.rb:180:in `block in instrument'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications/instrumenter.rb:24:in `instrument'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/notifications.rb:180:in `instrument'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/abstract_renderer.rb:88:in `instrument'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/template_renderer.rb:58:in `block in render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/template_renderer.rb:66:in `render_with_layout'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/template_renderer.rb:57:in `render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/template_renderer.rb:13:in `render'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/renderer.rb:61:in `render_template_to_object'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/renderer/renderer.rb:29:in `render_to_object'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/rendering.rb:118:in `block in _render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/base.rb:304:in `in_rendering_context'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/rendering.rb:117:in `_render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/streaming.rb:219:in `_render_template'
    from vendor/bundle/ruby/2.6.0/gems/actionview-6.0.1/lib/action_view/rendering.rb:103:in `render_to_body'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/rendering.rb:52:in `render_to_body'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/renderers.rb:142:in `render_to_body'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/abstract_controller/rendering.rb:25:in `render'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/rendering.rb:36:in `render'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/instrumentation.rb:44:in `block (2 levels) in render'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/core_ext/benchmark.rb:14:in `block in ms'
    from vendor/ruby-2.6.5/lib/ruby/2.6.0/benchmark.rb:308:in `realtime'
    from vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/core_ext/benchmark.rb:14:in `ms'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/instrumentation.rb:44:in `block in render'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/instrumentation.rb:85:in `cleanup_view_runtime'
    from vendor/bundle/ruby/2.6.0/gems/activerecord-6.0.1/lib/active_record/railties/controller_runtime.rb:34:in `cleanup_view_runtime'
    from vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_controller/metal/instrumentation.rb:43:in `render'
    from app/controllers/dealers/call_centre_controller.rb:11:in `index'
```

After some investigation it seems to me that there is some non-thread
safe code (we run single process, multi-thread instance of our apps in
production) in the rendering process.

Specifically in accessing the memoized `@view_context_class` variable
(which is what `ActionView::Template#compile` ends up adding methods
to). I've built a reproducible example at
CGA1123/action-view-race-condition specifically
CGA1123/action-view-race-condition@64c023081dd898be1f99c558e47726bc648a0374
which introduces some extra work on assignment in order to force the
ruby process to schedule another request thread.

The race means that we 'lose' the methods adding during a call to
`ActionView::Templace#compile!` but that instance still has `@compile`
set to `true` causing all subsequent attempts to render that template to
fail.

I'm not sure whether adding a mutex on the access is the best way to
solve this? Perhaps the caching logic should be smarter and check that
the method exists?

I'm also not sure of the performance implications of synchronizing
accessing this variable might have?

actionview/lib/action_view/lookup_context.rb

@@ -61,6 +61,7 @@ class DetailsKey #:nodoc:
 
       @details_keys = Concurrent::Map.new
       @digest_cache = Concurrent::Map.new
+      @view_context_mutex = Mutex.new
 
       def self.digest_cache(details)
         @digest_cache[details_cache_key(details)] ||= Concurrent::Map.new
@@ -89,7 +90,9 @@ def self.digest_caches
       end
 
       def self.view_context_class(klass)
-        @view_context_class ||= klass.with_empty_template_cache
+        @view_context_mutex.synchronize do
+          @view_context_class ||= klass.with_empty_template_cache
+        end
       end
     end