replication: do not stop syncing if replicas are loading

If a replica disconnects while sync is in progress, box.cfg{} may stop
syncing leaving the instance in 'orphan' mode. This will happen if not
enough replicas are connected to form a quorum. This makes sense e.g. on
network error, but not when a replica is loading, because in the latter
case it should be up and running quite soon. Let's account replicas that
disconnected because they haven't completed initial configuration yet
and continue syncing if connected + loading > quorum.

Closes #3422