Multiple riak_repl2_fscoordinator_sup workers per cluster when adding nodes

[nerophon]

A customer finds multiple fullsync coordinator workers running simultaneously on each of two clusters. This causes multiple fullsync schedules to run concurrently; the actual fullsync operations may or may not overlap, but each coordinator is active and has its own timer.

This state is reproducible as follows:

1. Set up two clusters, A & B.
2. Set up REPL and connect them (cluster manager 0.0.0.0:9080).
3. Set fullsync_on_connect to true (unclear whether this step is required).
4. Push continuous load onto cluster A.
5. Start fullsync with A as source and B as sink.
6. While fullsync is running, join one or more new nodes to A.
7. On all nodes riak attach and run supervisor:count_children(whereis(riak_repl2_fscoordinator_sup))..
8. Observe that worker count > 0 on more than one node. In my test, it was on the original coordinator and also the newly joined node.

The workaround for this issue is to manually kill all riak_repl2_fscoordinator_sup processes as follows:

1. stop & disable fullsync
2. wait a few minutes
3. on each node attach and run: Pid = whereis(riak_repl2_fscoordinator_sup). then erlang:exit(Pid,kill)..
4. wait a few minutes
5. enable & start fullsync

The symptoms of this issue are extremely slow fullsync operations, cluster overload / slowness, and fullsync activity in the logs when no fullsync ought to be running.

[nerophon]

Internal duplicate: https://github.com/basho/riak_repl/issues/748