ENG-6162: Check for in-progress-snapshot before issuing @SnapshotSave in
SnapshotDaemon.

If another is snapshot is still in progress somewhere in the cluster, don't
issue the next @SnapshotSave.
ENG-6162: Send feasibility check response back to SnapshotDaemon.

The response has to go back to SnapshotDaemon instead of going back to the
client directly so that whatever handling that needs to happen at the daemon
will happen.
Merge pull request #1305 from VoltDB/ENG-6162

ENG-6162: Check for in-progress-snapshot during feasibility check