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