Share thresholding state across Sentinels

One of the main challenges in supporting thresholding on Sentinel is sharing the internal thresholding state across the pool of Sentinels. Sentinels may be added or removed anytime, and we need to ensure that state of the thresholds are preserved.

Here is an example scenario to consider:

1. Sentinel A processes a CollectionSet with a relative-change threshold associated, the current value is 5
2. Sentinel A dies
3. Sentinel B processes the following CollectionSet for that same threshold with a value of 999, we expect the threshold to be triggered