MutablePropertySources uses an internal CopyOnWriteArrayList for defensiveness against concurrent modifications

Issue: SPR-12428
(cherry picked from commit 1ef06cc)
MethodParameter generally uses volatile variables where applicable now (as well as a local copy of the parameterNameDiscoverer field)

Issue: SPR-12453
(cherry picked from commit 7fcadaa)
Unit test for JavaBeans introspection against FreeMarker Configuration class

Issue: SPR-12448
(cherry picked from commit 282aded)
RequestMappingHandlerAdapter allows for overriding handleInternal

Issue: SPR-12460
(cherry picked from commit ece2c90)
Rearranged WebAsyncTask constructor interdependencies and assertions

Issue: SPR-12457
(cherry picked from commit aadb93f)
Polishing
(cherry picked from commit 2675ce7)