MB-8696: dump error msgs within XDCR if outpaced by db purger

MB-8012 implements the time-stamped based deletion purge in db.
Although the default behavior is to purge deletions older than
several days which is way longer than XDCR lag, unfortunately
there is no guarantee that XDCR is always ahead of the purger.
If XDCR is outpaced by db purger, at least one deletion has
has been lost and we need dump errors into logs to alert users.

Change-Id: I2ca01c71e1bf00c07535f66e84c9787f6b1f4d95
Reviewed-on: http://review.couchbase.org/27665
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>