Add cancellation support in TransportGetAllocationStatsAction

[JeremyDahlgren]

Replaces the use of a SingleResultDeduplicator by refactoring the cache as a subclass of CancellableSingleObjectCache. Refactored the AllocationStatsService and NodeAllocationStatsAndWeightsCalculator to accept the Runnable used to test for cancellation.

Closes #123248

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[elasticsearchmachine]

Hi @JeremyDahlgren, I've created a changelog YAML for you.

[DaveCTurner]

My feeling is that there has to be a simpler way to achieve what we want than adding all this bare-handed locking etc. I would have expected something based on ref-counting would be neater: each waiting listener should hold a ref, releasing it on cancellation, and an ongoing computation stops early if the number of waiting refs drops to zero.

[JeremyDahlgren]

My feeling is that there has to be a simpler way to achieve what we want than adding all this bare-handed locking etc. I would have expected something based on ref-counting would be neater: each waiting listener should hold a ref, releasing it on cancellation, and an ongoing computation stops early if the number of waiting refs drops to zero.

I applied your suggested change to onFailure() the listener right when cancellation is detected in CancellableSingleObjectCache, and refactored TransportGetAllocationStatsAction to use it. The changes are less complex now.