Add ping_quorum and get_peer_info support calls

`riak_ensemble_peer:ping_quorum/1` checks an ensemble for an active
quorum, returning the current ensemble leader as well as the list of
peers supporting the current quorum.

`riak_ensemble_manager:get_peer_info/2` returns information about a
given ensemble peer: the peer's current FSM state, if it is or is not
trusted, and it's current epoch.

Sibling pull-request: basho/riak_kv#979