Allow 1PCs for synchronous xsite replication

We should allow a 1PC optimization for xsite synchronous replication. Since synchronous replication is performed in the originating site's transactional context and the remote locks are acquired at prepare time, we can still guarantee consistency of data.

The default should be to use 1PC. However, this feature will require a new configuration option. <backup ... use2PC="true"/> to allow the traditional 2PC to be enabled.