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.
ENG-6242: Fix null timestamp sigil handling in ParameterConverter.
Merge pull request #1327 from VoltDB/ENG-6205

ENG-6205: Prevent back to back @SnapshotSave.
Fix a new test issue where we'd offer transactions in the test after
killing nodes before the repair process had finished.  Add a little
time after each node kill, and add a catch-all check in the
exceptional case to avoid false failures.
Merge pull request #1329 from VoltDB/ENG-6242

ENG-6242: Fix null timestamp sigil handling in ParameterConverter.

r=izzy, akhanzode