Added invalidationEvents method to publish cache invalidation events.

[ithinkicancode]

Resolves #44

[ithinkicancode]

A few notes:

1. Recommend using the Split (side-by-side) view to review changes. Most lines are caused by indentation. Git diff is really bad at this kind of change.
2. It's a draft because the performance of ChurnBenchmark has degraded by about 20%. It was worse in the beginning :) I have found using ZIO.when operator can degrade the performance on a hot path. I'm using regular if for now. (Might want to replace the when operator with if elsewhere too.)

[strokyl]

A few notes:

1. Recommend using the Split (side-by-side) view to review changes. Most lines are caused by indentation. Git diff is really bad at this kind of change.

Did you try "Hide whitespace" feature in github? It help a lot