Previously, we had several reimplementations of the same basiccaching mechanisms. In particular, `cleanAfterRun()`-based removalof caches not used in a given run.In this commit, we introduce common caching utilities. The providecommon implementations of the various idioms that we use. Thissimplifies all the use sites, which can now focus on their corelogic, instead of mixing it with caching mechanisms.The abstraction is not zero-cost everywhere. It may introduce someconstant overhead.

This simplifies the emitter's `KnowledgeGuardian`, which can nowfocus on its core logic without handling registrations andinvalidations.