Added a trivial implementation of the LeadershipService.

Also renamed SdnIpLeadershipService to HazelcastLeadershipService, and moved
it into the distributed core bundle.

This allows applications which depend on LeadershipService to be used with
the trivial core.

Change-Id: Ie71a946d95653a4d2209afd3af0e7f23b5a4f818
4 files changed