Reallocate TT on threadpool resize.

Reallocate TT on threadpool resize.

Makes sure the potential benefit of first touch does not depend on
the order of the UCI commands Threads and Hash, by reallocating the
hash if a Threads is issued. The cost is zeroing the TT once more
than needed. In case the prefered order (first Threads than Hash)
is employed, this amounts to zeroing the default sized TT (16Mb),
which is essentially instantaneous.

Follow up for https://github.com/official-stockfish/Stockfish/pull/1601
where additional data and discussion is available.

Closes https://github.com/official-stockfish/Stockfish/pull/1620

No functional change.