MB-100 Log couchdb messages only to the dedicated file.

Sometimes couchdb may be particularly loud which causes logs to be rotated
much faster than we would want.

Change-Id: I4020010e1175ce03d9b17d43002b5a141ff8b7bb
Reviewed-on: http://review.couchbase.org/26348
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>