[KafkaIO] Improve caching in backlog estimation and processing

[sjvanrossum]

Caching improvements in a number of places:

• Migrated per instantiation caches to use the new MemoizingPerInstantiationSerializableSupplier utility class.
• Added a per instantiation cache for processing consumers.
• Moved end offset cache refreshes to a background thread.

This results in a significant reduction of processing consumers (before: consumer per split per process element per worker, after: consumer per split per worker). Processing consumers are evicted if they're eligible for collection and are more likely to close their connections after connections.max.idle.ms.

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[chamikaramj]

Any updates ?

[chamikaramj]

@kennknowles @shunping or @Dippatel98 PTAL

[kennknowles]

ACK - this change will take me some time to grok

[kennknowles]

@johnjcasey

[github-actions]

Reminder, please take a look at this pr: @kennknowles @shunping @Dippatel98

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[ahmedabu98]

Hey @sjvanrossum, are you still working on this?

[sjvanrossum]

@ahmedabu98 I just pushed a requested docs change and a fix for a bug I spotted earlier, otherwise this is ready.

[sjvanrossum]

@johnjcasey @ahmedabu98 I've addressed the integration test failure in #34711.

[sjvanrossum]

@johnjcasey it looks like #34711 fails as a separate PR, but succeeds as part of this PR so I've added it here. Would you mind double checking if it makes sense to have roundTripElements await and then cancel the pipelines it creates?

My understanding of that test is that we validate that a consumer will resume from a committed offset, but waitUntilFinish doesn't guarantee that the pipeline is finished when the timeout expires so both pipelines may be trying to make progress at the same time.