Not check for reachability for non-continuous replication

* Rework per code review

* Not check for reachability for non-continuous replication as
the reachability for non-existent host sometimes first report
that the host is unreachable, then a moment later say it's reachable.
Merge pull request #556 from couchbase/feature/issue_544

#544 Don't check for reachability for non-continuous replication