R1:731404445fe8

Improve task subpriority movement algorithm for homogenous blocks

Summary:
Fixes T7664. When there are a large number of tasks (400+) with the same subpriority (which can happen if the subpriority features are rarely used), it may take more than 30 seconds to rebalance them.

Make the algorithm more aggressive about rebalancing homogenous blocks of tasks.

This may need to get even fancier, but I'd guess it can process blocks 1-2 orders of magnitude larger, which should be ~all installs.

(If someone…