ENG-5844: Remove site from currently snapshotting set before logging completion
to ZK.

Otherwise it's possible that the completion interests will fire before the sites
remove themselves from the set.
ENG-5844: Remove the node from currently snapshotting ZK node before the site
removes itself from the set.
Merge pull request #1267 from VoltDB/ENG-5844-fixes

ENG-5844: Snapshot race fix