Cache operation invocation hook point

This commit adds a invokeOperation protected method in case one
needs a hook point in the way the underlying cache method is invoked,
and how exceptions that might be thrown by that invocation are handled.

Issue: SPR-11540
Clarify the use of sessionTransacted

This commit clarifies the role of the external transactionManager for
a DMLC and the relationship with the sessionTransacted flag

Issue: SPR-9200