MINOR: check: also consider the random other thread's active checks

MINOR: check: also consider the random other thread's active checks

When checking if it's worth transferring a sleeping thread to another
random thread, let's also check if that random other thread has less
checks than the current one, which is another reason for transferring
the load there.

This commit adds a function "check_thread_cmp_load()" to compare two
threads' loads in order to simplify the decision taking.

The minimum active check count before starting to consider rebalancing
the load was now raised from 2 to 3, because tests show that at 15k
concurrent checks, at 2, 50% are evaluated for rebalancing and 30%
are rebalanced, while at 3, this is cut in half.