Make txn_factory an attribute of the base class.

Raise an exception when prepare() returns False, rather than
automatically aborting.

Pass transaction object to Rollback() so that rollback() method can
check state of transaction.

Add IllegalStateError calls to prevent assertions from failing.
XXX Should the manager duplicate these checks?

Add suspend() and resume() to non-threaded txn manager.

Fix bug that caused threaded suspend() to fail with KeyError for
thread with no current transaction.