Rework determineTransactionManager condition

SPR-11954 introduced a regression that when the "default" transaction
manager is cached, qualified transaction managers are not taken into
account anymore.

This commit rework the "determineTransactionManager" condition to
favor qualifier and "named" transaction managers. If none of these apply,
the default transaction manager is used as it should.

Also reworked the caching infrastructure so that a single cache holds
all transaction manager instances.

Issue: SPR-12541
Do not retain cache transaction managers

Previously, cache transaction managers may be  retained outside the
boundaries of an application context with AspectJ since an aspect is
basically a singleton for the current class loader.

This commit adds a "clearTransactionManagerCache" that is similar to the
"clearMetadataCache" introduced in CacheAspectSupport: whenever the
context is disposed, the cache is cleared to remove any reference to a
transaction manager defined by that context.

Issue: SPR-12518