Ruby 2.7 warning: creating a Proc without a block

This reverts commit 3420a145, reversing
changes made to afb66a5a.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The issue is that on the exit from Instrumenter#instrument section,
an Evented listener will run into an error because its thread local
(Thread.current[:_timestack]) has not been set up by the #start
method (this obviously happens because the Evented listeners didn't
exist at the time, since no subscribtion to that section was made yet).

Note: support for subscribing to instrumented sections, while being
inside those instrumented sections, might be removed in the future.

Maybe fixes #21873.

The thread_safe gem is being deprecated and all its code has been merged
into the concurrent-ruby gem. The new class, Concurrent::Map, is exactly
the same as its predecessor except for fixes to two bugs discovered
during the merge.

Summary of the changes:
 * Add thread_safe gem.
 * Use thread safe cache for digestor caching.
 * Replace manual synchronization with ThreadSafe::Cache in Relation::Delegation.
 * Replace @attribute_method_matchers_cache Hash with ThreadSafe::Cache.
 * Use TS::Cache to avoid the synchronisation overhead on listener retrieval.
 * Replace synchronisation with TS::Cache usage.
 * Use a preallocated array for performance/memory reasons.
 * Update the controllers cache to the new AS::Dependencies::ClassCache API.
   The original @controllers cache no longer makes much sense after @tenderlove's
   changes in 7b6bfe84 and f345e238.
 * Use TS::Cache in the connection pool to avoid locking overhead.
 * Use TS::Cache in ConnectionHandler.

In cases where a notification subscriber includes methods to support
both Evented and Timed events, Evented should take priority over Timed.
This allows subscribers to be backwards compatible (older Rails only
allows Timed events) while defaulting to newer behavior.

* Refactor log subscriber, use select! to avoid a new object
* Remove deprecation messages related to AS::Deprecation behavior
  This was added about 2 years ago for Rails 3:
  https://github.com/rails/rails/commit/d4c7d3fd94e5a885a6366eaeb3b908bb58ffd4db
* Remove some not used requires
* Refactor delegate to avoid string conversions and if statements inside each block

Performance optimizations to handle cases of instrumentors that are not listened to. Also, fix a possible concurrency issue.

Minor performance improvment in notifications/fanout and active_record/log_subscriber [#5098 state:open]

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Optimize AS::Notifications to remember which subscribers don't match and not run them. This will allow notifications that are only useful in dev or testing to run efficiently in production.