MB-100 Don't use couch_config where it makes no sense.

Particularly:
  1. don't ask for replicator db name because in many places we just
  assume its name is _replicator anyway
  2. currently our replications are always continuous and xdcr code
  doesn't even bother to check the value of 'continuous' parameter; so
  I got rid of it too

Change-Id: I6ceb802329bb678c435c51706784ef5c904cefe1
Reviewed-on: http://review.couchbase.org/28167
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>