Merge pull request #6398 from pmahoney/threadsafe-connection-pool

Synchronize read and modification of @reserved_connections 

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -139,15 +139,19 @@ def initialize(spec)
       # #connection can be called any number of times; the connection is
       # held in a hash keyed by the thread id.
       def connection
+        synchronize do
           @reserved_connections[current_connection_id] ||= checkout
         end
+      end
 
       # Is there an open connection that is being used for the current thread?
       def active_connection?
+        synchronize do
           @reserved_connections.fetch(current_connection_id) {
             return false
           }.in_use?
         end
+      end
 
       # Signal that the thread is finished with the current connection.
       # #release_connection releases the connection-thread association