Clarify some comments in SMP code

Spotted by Joona.

No functional change.
Pass Position as const ref in update_stats()

No functional change.
Make imbalance table more clear

No functional change.
Retire "Idle Threads Sleep" UCI option

After last Joona's patch there is no measurable
difference between the option set or unset.

Tested by Andreas Strangmüller with 16 threads
on his Dual Opteron 6376.

After 5000 games at 15+0.05 the result is:

1 Stockfish_14050822_T16_on   : 3003  5000 (+849,=3396,-755), 50.9 %
2 Stockfish_14050822_T16_off  : 2997  5000 (+755,=3396,-849), 49.1 %

bench: 880215
Save stalemates in TT

When there aren't legal moves after
a search, instead of returning imediately,
save bestValue in TT as in the usual case.

There is really no reason to special case
this one.

With this patch is fully fixed (again) follwing
position:

    7k/6p1/6B1/5K1P/8/8/8/8 w - - 0 1

Also in SMP case.

bench: 8802105