Improve logging in TransactionalTEL

This commit makes the logging in TransactionalTestExecutionListener
consistent for both starting and ending transactions. Specifically,
the current TestContext is now included in the informational log
statement when starting a new transaction.

Issue: SPR-11323
Polish log messages
Move "handlers" field to AbstractSubscribableChannel

Move the management of subscribers to the abstract parent class where
it belongs.
Make TypeDescriptor more amenable to subclassing

Change the previously package scope TypeDescriptor constructor to
protected and add a getResolvableType() protected method.

Issue: SPR-11303