For 1-Wesolowski proofs (used for "blueboxing") the squaring thread
previously needlessly continued squaring while proof computation was in
progress. In practice, proof computation also takes a while, so this
could easily result in 10-20M additional squaring iterations. With this
change, the squaring stops after the iterations target is reached. This
also makes CPU utilisation more predictable. Previously, during squaring
2 threads were heavily utilised, and 3 threads during proof computation.
Now it's never more than 2 high-load threads: 2 for squaring, 1 for
proof computation.