Use calloc() in TranspositionTable::set_size()

Function calloc() already initializes memory to
zero, so avoid calling clear() afterwards.

Also some renaming while there (inspired by DiscoCheck).

No functional change.
Reorder conditions according to their frequency

This should minimize useless tests.

No functional change.
Simplify search results update

Also some rename while there.

No functional change.
Fix a stale comment

No functional change.