Remove unused parameter requestId from SnapshotDaemon.initiateSnapshot().
ENG-6205: Handle snapshot feasibility check timeout properly.
ENG-6205: Prevent calling @SnapshotSave back to back.

Track the previous call to @SnapshotSave so that when a new snapshot is
requested, the SnapshotDaemon won't issue another one if the previous one hasn't
responded.

It's okay if the previous one is still in progress, the feasibility check will
take care of that. We just care about queued but not executed @SnapshotSave.
Merge pull request #1327 from VoltDB/ENG-6205

ENG-6205: Prevent back to back @SnapshotSave.