CASSANDRA-15510 (4.0): Optimise BTree.{build,update,transform}

[blerer]

[belliottsmith]

Called it nonEmptyParentMaybeSteal and changed touch to inUse. I didn't reset it to false to avoid risking uncleaned arrays after use.

I did mentally wonder about this after proposing it (without checking) and figured if that were the case we could easily introduce two flags used (or touched) and inUse - set both on touch and unset only inUse when empty.

[belliottsmith]

Modulo rolling that one errant change I suggested back, LGTM +1

[blambov]

Rollback pushed to the same branch.

[belliottsmith]

Perfect. I think I broke Jira with all my worklog updates - at least, I cannot load the page successfully now - so I'll just leave my +1 here.