ENG-6205: Prevent concurrent snapshot feasibility checks.

If two snapshot feasibility checks run concurrently, they will both see that
there is no snapshot in progress and call @SnapshotSave. Then the collision
happens inside @SnapshotSave.

Put a guard in SnapshotDaemon to prevent two feasibility checks to occur at the
same time.
Merge pull request #1309 from VoltDB/ENG-6205

ENG-6205: Prevent concurrent snapshot feasibility checks.