Add documentation for the keep-alive flag

This commit updates the documentation of the task:executor element to
reference the keep-alive flag.

Issue: SPR-12407
(cherry picked from commit 6534d00)
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
(cherry picked from commit da04362)
Improve exception message

Issue: SPR-12230
(cherry picked from commit 7f4bf41)