Fully concurrent reads design proposal

[jpbetz]

Fully Concurrent Reads Design Proposal

Summary: The etcd code base has all the fundamental building blocks to support fully concurrent reads, but currently long running ("expensive") reads can cause head-of-line blocking.

Authors: @jingyih, @jpbetz

Looking for feedback on the design as well as on the WIP prototype: #10523

[jpbetz]

cc @xiang90 @heyitsanthony @gyuho

[stale]

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