Make backupIndexes actually useful

- Cleanup code all over the place, use "snapshot" consistently since
  that's what they're called
- Implement snapshot deletion
- Remove --all option, it's rather insane
- Only work with one repo at a time, iterating is pretty crazy
- Drop timestamp from default backup name. Otherwise we don't get the
  benefit of being incremental.

Change-Id: I3265308d9d764d589c764ef8333e90fe982830be
Merge "Make backupIndexes actually useful"