[WIP] Add background thread and caching of consumers to Kafka SDF

[scwhittle]

Attempt to improve SDF throughput to UnboundedReader impl by similarly performing blocking kafka polls in background thread and is cached.
This allows main partition consuming thread to be blocked less, prefetches, and caching reduces consuemr connections.

Remaining work:

• figure out how to address when a read is stopped due to splitting. Currently records that were pulled off of background queue are lost. Could perhaps be added back to the background queue in some way or we could change the interface to internally vend a record at a time and keep track of if it is consumed or not.
• unit testing and performance testing
• some of the timeout configuration plumbing needs to be fixed

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[scwhittle]

R: @bzablocki

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[bzablocki]

Thanks, Sam! I tested this change, and it seems to make the throughput "smoother", but not necessarily higher. The average throughput remains about the same.

[scwhittle]

Thanks for testing Bartosz! I'll clean it up and add tests. I think it will be helpful for preventing the high # of consumer creations which I believe was affecting some kafka sdf users.

[github-actions]

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[bzablocki]

I'm continuing the work on this in my PR (#32597).