Reduce PlatformTransactionManager lookups

Prior to this commit, cache operations mentioning a qualifier led to
a lookup for the same PlatformTransactionManager over and over again.
The same applied when a transactionManager bean name was specified on
the interceptor.

This commit adds a cache to store the reference of such transaction
managers. As a convenience, the default PlatformTransactionManager is
also initialized if it has not been through configuration.

Issue: SPR-11954