MODE-1968 Improved/simplified logging of persisted changes

When a session is saved, the WritableSessionCache now outputs
more details of the changes to the 'org.modeshape.jcr.txn' logging
context under TRACE logging. This logging context does not match
the normal conventions, but it makes it really easy to turn on
TRACE logging on this context and see all the save- and transaction-
related information.
MODE-1968 Corrected test case