Fully joined peer should attempt to rejoin after seeing its own leave-cluster

[lbradstreet]

Currently unhandled. See https://github.com/onyx-platform/onyx/blob/0.8.x/src/onyx/log/commands/leave_cluster.clj#L53

I believe the peer should either emit an abort-join-cluster, as above, or close the restart-ch for the peer and become a whole new peer, with a whole new peer-id. I think the abort-join-cluster is OK, but there may be cases where the latter is safer.

[lbradstreet]

Closed by #484