ENG-4721: Mark a snapshot as truncation snapshot if it's in the command log
snapshot directory.
ENG-4721: Pass snapshot path to SnapshotCompletionEvent.
ENG-4721: Add valid snapshot information to m_truncationSnapshotAttempts in
SnapshotDaemon.

Without the path and the nonce, the snapshot daemon won't know how to groom the
truncation snapshot later.
Merge pull request #586 from VoltDB/ENG-4721

ENG-4721: Groom truncation snapshots after elastic join