MB-10194: Make sasl_auth() and open_connection() externally usable

XDCR wants to handle the connection stuff itself instead of using
couch_upr:start/2. Hence the sasl_auth() and open_connection()
functions were modified and exported so that they can be used
from the outside.

Change-Id: I15c68f3b20dac70fa7c128b5a9d400d6cee235ea
Reviewed-on: http://review.couchbase.org/33505
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>