Allow consolidating config in root context with Java

This change makes it possible to provide no configuration for the
DispatcherServlet when extending
AbstractAnnotationConfigDispatcherServletInitializer, and therefore
provide all config through the "root" context.

Issue: SPR-11357
Improve 'switch off commons-logging' documentation

Update reference documentation to make it clearer that only
`spring-core` has a direct dependency on `commons-logging`.

Also reference the 'empty jar' alternative option as described in
the SLF4J FAQ.
Merge pull request #445 from snicoll/commons-logging-doc

* commons-logging-doc:
  Improve 'switch off commons-logging' documentation
Added test for SpringProperties setFlag method

Issue: SPR-9014
Issue: SPR-11297
(cherry picked from commit 23249cd)
Drop Introspector.flushFromCaches calls completely

According to the JDK's documentation and changelog, the Introspector itself safely handles weak references as of JDK 6 update 21 (which is what we require for Spring 4.0).

Issue: SPR-11356
Polishing

Issue: SPR-11357