This method suppose to watch for changes of leader key if current node
is not leader and also it could watch for changes in a members list if
current conde is the leader.
`Cluster.leader` is not reference to `Member` anymore, but to `Leader`
`Leader` class contains field `index` (update index). This field is very
useful for watching for events which changing leader key. Also `Leader`
contains `member` field, which should reference real member.
List of ZooKeeper nodes could be periodically updated from Exhibitor
Since we know that each Exhibitor accompanies one ZooKeeper node, list
of Exhibitor nodes also maintained. Exhibitor assumes that all ZooKeeper
nodes are using the same client port, 2181. The same assumption is valid
for Exhibitor, it should always listen on the same port on all nodes.
Original list of Exhibitor nodes is cached and used as a fallback when
it failed ito query information with using maintained list.
This implementation is using the same interface (AbstractDCS) as Etcd
class. It means that there should be no problem to implement another
plugin to work agains Consul for example.
Alexander Kukushkin