log xdcr replication stint starts and finishes

Change-Id: Ib24a62a14fb4e3a9c0b9e6dcaa6e5272bd293a83
Reviewed-on: http://review.couchbase.org/42534
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>