Log context cache statistics in the TCF

Prior to this commit, finding out how many application contexts had
been loaded within a test suite required the use of reflection and a
bit of hacking.

This commit addresses this issue by logging ContextCache statistics
whenever an application context is loaded by the Spring TestContext
Framework (TCF).

The log output can be enabled by setting the
"org.springframework.test.context.cache" logging category to DEBUG.

Issue: SPR-12409
Explicitly mention DelegatingWebMvcConfiguration in @EnableWebMvc Javadoc
Add descriptions to Gradle tasks in spring-test
Improve exception message

Issue: SPR-12230
Fix failing test
Straight JmsMessagingTemplate setup based on JMS ConnectionFactory

Issue: SPR-12413
Polishing
Avoid unnecessary synchronization in ContextCache, plus forward-ported polishing

Issue: SPR-12409
ShallowEtagHeaderFilter skips "Cache-Control" header check on Servlet 2.5

Issue: SPR-12414
Latest dependency updates (Tomcat 8.0.15, Jetty 9.2.4, Protocol Buffers 2.6.1)
Polishing
Polish Javadoc for ContextCache
Check STOMP headers against ending backslash

Issue: SPR-12418