Replicaset ipv6 fix

replicaset/replicaset: fix ipv6 addresses in and out of replicaset code

Mongo only works with "bad" ipv6 addresses (::1:port format), we would rather use the "good" format ([::1]:port). This change makes sure that we only use (and see) "good" format addresses outside of the replicaset code, but we still send bad format addresses to mongo.