Z3 PostgreSQL database adapter does not do two-phase commit

With PostgreSQL 8.1, we can do a proper two phase commit. The existing PostgreSQL database driver does not do this, as the feature did not exist when it was written.

We should update the PostgreSQL database adapter for Zope3 to do a proper two phase commit. Because we are now using multiple data stores (the session database and the primary database), this is important and possibly a dataloss bug. At a minimum, it will cause occasional errors to end users. For example, every time a serialization exception occurs with the session database, changes to the primary database may have already been committed so when Z3 retries the transaction an error will likely be raised.