cross-database transaction manager

[gburgett]

Currently, the ThreadContextTransactionManager is bound to one instance of XFlatDatabase. We could conceivably extricate it enough that it could handle multiple databases, providing cross-database transaction management.

This is possible since the engines themselves bind to the transaction manager. The only place where they are intertwined is in the durability code, particularly recovery. We could change the journaling to keep track of which databases are bound to a transaction as well as which engines, so we can revert those transactions whenever the engines spin up.