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