Use exceptions to stop the search

Instead of classical flags, throw an
exception when we want to immediately halt
the search. Currently only one type
is used for both UCI stop and threads
cut off.

No functional change.
Some renaming in TT store()

No functional change.
Simplify DistanceRingsBB init

Verified by same benchmark and picking some random values.

No functional change.
Revert using exceptions

Due to crashes. It will be reapplied once
we understand what's happening.

No functional change.