++timestamp to MembershipEvent

[artem-v]

Motivation:

Facilitate debugging and monitoring of cluster events

Updates:

• Added method long MembershipEvent.timestamp() which returns UTC milllis of when event was created.
• Output of MembershipEventsExample:

23:27:01.147 Alice join members: [Alice/a71d806f184d16@127.0.0.1:37188]
23:27:01.473 Alice received: MembershipEvent[type=ADDED, member=Bob/692a1b23e0674abc@127.0.0.1:42298, oldMetadata=null, newMetadata=bfddad89-14, timestamp=2019-09-21T20:27:01.473Z]
23:27:01.490 Bob received: MembershipEvent[type=ADDED, member=Alice/a71d806f184d16@127.0.0.1:37188, oldMetadata=null, newMetadata=af674fde-16, timestamp=2019-09-21T20:27:01.490Z]
23:27:01.491 Bob join members: [Bob/692a1b23e0674abc@127.0.0.1:42298, Alice/a71d806f184d16@127.0.0.1:37188]
23:27:01.504 Alice received: MembershipEvent[type=ADDED, member=Carol/35f3111be5de4eb5@127.0.0.1:37544, oldMetadata=null, newMetadata=d29a258d-16, timestamp=2019-09-21T20:27:01.504Z]
23:27:01.504 Bob received: MembershipEvent[type=ADDED, member=Carol/35f3111be5de4eb5@127.0.0.1:37544, oldMetadata=null, newMetadata=d29a258d-16, timestamp=2019-09-21T20:27:01.504Z]
23:27:01.507 Carol received: MembershipEvent[type=ADDED, member=Bob/692a1b23e0674abc@127.0.0.1:42298, oldMetadata=null, newMetadata=bfddad89-14, timestamp=2019-09-21T20:27:01.507Z]
23:27:01.508 Carol received: MembershipEvent[type=ADDED, member=Alice/a71d806f184d16@127.0.0.1:37188, oldMetadata=null, newMetadata=af674fde-16, timestamp=2019-09-21T20:27:01.508Z]
23:27:01.508 Carol join members: [Bob/692a1b23e0674abc@127.0.0.1:42298, Carol/35f3111be5de4eb5@127.0.0.1:37544, Alice/a71d806f184d16@127.0.0.1:37188]
I 0921-2327:01,512 i.s.c.Cluster Cluster member Bob/692a1b23e0674abc@127.0.0.1:42298 is shutting down [main]
23:27:01.556 Alice received: MembershipEvent[type=REMOVED, member=Bob/692a1b23e0674abc@127.0.0.1:42298, oldMetadata=bfddad89-14, newMetadata=null, timestamp=2019-09-21T20:27:01.556Z]
23:27:01.556 Carol received: MembershipEvent[type=REMOVED, member=Bob/692a1b23e0674abc@127.0.0.1:42298, oldMetadata=bfddad89-14, newMetadata=null, timestamp=2019-09-21T20:27:01.556Z]
I 0921-2327:02,968 i.s.c.Cluster Cluster member Bob/692a1b23e0674abc@127.0.0.1:42298 has been shut down [sc-cluster-42298-2]