ENG-7019: Fix Race Condition in Stream Snapshot Watchdog

Ideally, the data structure used to track outstanding work can be replaced, but that introduces more changes. This change set has lower risk. It is potentially back portable to 3.x releases.

Jenkins is still running tests, don't merge yet.
Yes. This is actually the key to this fix. The synchronization on the run method is completely unnecessary because the other accesses to the map is synchronized on the parent StreamSnapshotDataTarget object.