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
fd7153ffbb