Improve rule/filter caching mechanism

[FoxxMD]

The current caching mechanism uses a scoped (per event) Map of results to return early. This is fine but at the same time CM is still generating, and keeping in memory, "duplicate" rule/filters that are never used due to the cached results.

The caching mechanism can be simplified, and rules/filters can be de-duplicated, by combining the two using a memoize promise.