Previously an addAfterCommitHook was used, but this is not run on transaction
aborts.  Now a Synchronizer is used which unconditionally closes the request
after a transaction is finished.

Not closing the request seems to have caused the database connections to be
kept open and these nasty log messages:

    CRITICAL ZODB.DB DB.open() has 42 open connections with a pool_size of 7

42 being the wrong answer in this case.