Artifact replication survives peer restart

[kevstigneev]

If either primary Nexus (that gets an artifact upload) or a replica instance is restarted after an artifact is uploaded to the primary instance, the artifact is replicated after the restart.

Expected results:

• If the primary Nexus instance is restarted before it sends the notification to peers, it resends it after restart. Possibly - to all peers (then repeated notifications are gracefully handled by the replica) or just to those that didn't get it.
  • If a replica got a notification from a primary instance but did not completed the artifact download and is restarted, it downloads the artifact after the restart.
  • If primary or replica Nexus is restarted during replication, the artifact is re-downloaded after the restart.

It's a fault tolerance extention for #4 and #5.