fullsyncs_completed should not be held in FSM state

[binarytemple]

fullsyncs_completed is not held in a global location, it is passed on the stack.

{fullsyncs_completed, State#state.fullsyncs_completed},

If the riak_repl2_fscoordinator crashes (as it often does), the metric is lost.

This is causing customer confusion, as the metric keeps being reset to zero.

https://github.com/basho/riak_repl/blob/develop/src/riak_repl2_fscoordinator.erl#L242,L272

It would be better if it were stored in some global state, an ETS table or similar.

[engelsanchez]

An ETS table will only help up until the point where the leader changes and the coordinator processes start in another node. That could be a temporary patch, but things like this stored in cluster metadata would be nice.

[binarytemple]

Good point