Restore compatibility with eXist-db 2.1

After the last stable version of eXist-db the internal API has been changed, on which the replication extension depends to prevent re-replication of data (#30). For eXist-db 2.1 this protection is now effectively switched off, which is not a problem when using the extension in a classic master-slave(s) configuration(s).

Note: older versions of the extension can cause database corruption (2.1 only) because an Error occurred in the eXist-db Trigger mechanism. Please upgrade ASAP to this version

The change hasn't been extensively tested due to timing issues.

Detailed change log via 0.6.1...0.7