RCBC-168 DNS SRV helper for connection constructor

Change-Id: I4dab71bae075c41965615c8ca60ffafb915a299a
Reviewed-on: http://review.couchbase.org/36009
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>