MEDIUM: checks: search more aggressively for another thread on overload

MEDIUM: checks: search more aggressively for another thread on overload

When the current check is overloaded (more running checks than the
configured limit), we'll try more aggressively to find another thread.
Instead of just opportunistically looking for one half as loaded, now if
the current thread has more than 1% more active checks than another one,
or has more than a configured limit of concurrent running checks, it will
search for a more suitable thread among 3 other random ones in order to
migrate the check there. The number of migrations remains very low (~1%)
and the checks load very fair across all threads (~1% as well). The new
parameter is called tune.max-checks-per-thread.