move remote cluster service into xdcr

Change-Id: I242349dbe4d3416bc2ddb2786919a0f1e602d353
Reviewed-on: http://review.couchbase.org/43522
Reviewed-by: Xiaomei Zhang <xiaomeizhangatwork@gmail.com>
Tested-by: Xiaomei Zhang <xiaomeizhangatwork@gmail.com>
Reviewed-by: Yu Sui <ysui68@gmail.com>
Tested-by: Yu Sui <ysui68@gmail.com>