Port sticky assignor fixes (KAFKA-12984) back to 2.8

We should make sure that fix #1 and #2 of #10985 make it back to the 2.8 sticky assignor, since it's pretty much impossible to smoothly cherrypick that commit from 3.0 to 2.8 due to all the recent improvements and refactoring in the AbstractStickyAssignor. Either we can just extract and apply those two fixes to 2.8 directly, or go back and port all the commits that made this cherrypick difficult over to 2.8 as well. If we do so then cherrypicking the original commit should be easy.

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