ENG-4653, ENG-4735: Add initial OpsRegistrar implementation.  Unused,
goes nowhere, does nothing.
ENG-4653, ENG-4735: Consolidate construction, shutdown, dispatch of
various OpsAgents to use OpsRegistrar.
ENG-4653: Initial port of SnapshotScan system procedure into OpsAgent
world.  Currently unused, uncallable.
ENG-4653: Dispatch to the new SnapshotScan implementation.  Delete the
old one.
ENG-4731: These aren't really errors, we can dial down the log level.
ENG-4653: Move @SnapshotDelete to be a non-blocking OPS agent rather
than an MP system procedure (even though the actual deletion was done
under a separate thread, which I have not changed yet).
ENG-4653: Cleanup.  Unmagic some constants, add some commentary, etc.
ENG-4653: Code review fix.  Use generics correctly for the win.
ENG-4653: More review feedback: Use EnumMap
ENG-4653: Code review cleanup: Better empty string checkage.
Merge pull request #534 from VoltDB/eng4653

Eng4653