ENG-4586: Refactor: separate DR-specific stats collection work from
generic stats messaging code.
ENG-4586: Make the number of tables returned for the stat selector
flexible, albeit super-fragile.
ENG-4586: Start adding SNAPSHOTSTATUS to non-blocking stats collection
ENG-4586: Make test stop pretending to aggregate multiple stats
agents.  That much broken probably with IV2.
ENG-4586: Final bits:
- Make @SnapshotStatus redirect to the right @Statistics call
- Make old @SnapshotStatus sysproc fail hard (can't easily get rid of it as
long as the user can call @SnapshotStatus)
- Add SNAPSHOTSTATUS selector to statistics in SQLCommand
- Add a test that demonstrates a successful fix
ENG-4586: Review fixes:
- Add missing test assertions
- Delete stale SnapshotStatus sysproc IDs
Merge pull request #480 from VoltDB/eng4586

Eng4586: Fix @SnapshotStatus, r=ssantoro