Stop search if only 1 legal move

There is no point searching a move that is forced.
It wastes time while allowing computer opponents to
fill hash with 100% accuracy.

[edit: Condition moved together with "easy move" ones]

Bench identical: 4922272
Fix race condition where idle_loop() gets called from Split()

SplitPoint member slavesMask wasn't read under lock

No functional change.
Avoid locking/unlocking in a tight loop

After previous patch if split point master is
waiting for job and "Use Sleeping Threads" is
false (our condition for official releases) then
it will lock/unlock splitPoint mutex in a super
tight loop badly affecting performance.

Rewrite the code to lock only when we are about
to finish. Note that race condition on slavesMask
is anyhow fixed.

No functional change.
Simplify "easy move" detection

Detect a move as easy only if it is the only one ;-)
or if is much better than remaining ones after we
have spent 20% of search time.

Tests are ongoing, but it seems this semplification
stands. Anyhow it is experimental for now and could
be reverted/improved with further work Gary is
testing right now.

No functional change.
Revert "Store moves sent with "position" UCI command"

This reverts commit 0d68b523a390e2f5c37f211316869d798e852289.

After easy move semplification this machinery is not
needed anymore (because of we don't need to know if a
root move is a recapture)

No functional change.
Increase see prune depth

This seems good at short TC controls.

After 10000 games at 20+0.05
ELO: 9.56 +-6.8 (95%) LOS: 100.0%
Total: 10000 W: 1949 L: 1674 D: 6377

Testing at long TC and regression testing is still
ongoing. So this is a bit speculative commit and
could be reverted in the future.

Also re-testing at long TC the SEE pruning in PV nodes
seems less effective (perhaps even a regression, but
still ongoing) so disabled for now.

bench: 4968764
Merge branch 'master' of https://github.com/mcostalba/Stockfish