IntervalTree build throughput is low enough to be a bottleneck

With several terabytes of data and 8 compactors it’s possible for the compactors to spend a lot of time blocked waiting on IntervalTrees to be built.

There is also a lot of wasted CPU because it’s updated optimistically so most of them end up being thrown away.

This can end up being quite painful because it can block memtable flushing as well and then a single slow CFS can block unrelated CFS because the memtable post flush executor is single threaded and shared across all CFS.