ISynchronizer grows a newTransaction() method, called
whenever TransactionManager.begin() is called.

Connection implements that, and changes its ISynchronizer
afterCompletion() method, to call sync() on its storage
(if the storage has such a method), and to process
invalidations in any case.

The bottom line is that storage sync() will get done "by
magic" now after top-level commit() and abort(), and after
explicit TransactionManager.begin().  This should make it
possible to deprecate Connection.sync(), although I'm not
doing that yet.  Made a small but meaningful start by
purging many sync() calls from some of the nastiest ZEO
tests -- and they still work fine.