scheduler: Let ThreadData::ShouldRebalance() choose the actual core

Currently, ThreadData::ShouldRebalance() (and mode specific functions
it calls) only decides whether to migrate thread to another core or not.
However, in most cases it actually needs to find the best candidate for
new core so it could as well return that information.
scheduler: Fix and simplify thread migration logic in low latency mode

This patch remove the old thread migration logic which used few special
cases and (broken) general check that attempted to balance threads.

The new logic is pretty straightforward and seems perform well without
any additional special cases. Current core is compared with the least loaded
one and the thread is migrated if that would result in estimated loads of
both cores (i.e. the current one and the least loaded one) to become closer
to the average load (i.e. average of that two cores).