replication: fix rebootstrap race that results in broken subscription

While a node of the cluster is re-bootstrapping (joining again),
other nodes may try to re-subscribe to it. They will fail, because
the rebootstrapped node hasn't tried to subscribe hence hasn't been
added to the _cluster table yet and so is not present in the hash
at the subscriber's side for replica_on_applier_reconnect() to look
it up.

Fix this by making a subscriber create an id-less (REPLICA_ID_NIL)
struct replica in this case and reattach the applier to it. It will
be assigned an id when it finally subscribes and is registered in
_cluster.

Fixes 71b33405 replication: reconnect applier on master rebootstrap