MB-12210: log initial xdcr replication position to xdcr log

Change-Id: I045aaf78e6362cf9f511de26ef4c21c6887cacaa
Reviewed-on: http://review.couchbase.org/42530
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>