[cache] Revise the hash table accounting.

Instead of counting entries relative to the middle of the hash table,
this switches to the absolute counter with the full index range mask.
As a result, some calculations become a bit simpler.

* src/cache/ftccache.h (FTC_NODE_TOP_FOR_HASH): Revised.
* src/cache/ftccache.c (ftc_get_top_node_for_hash): Ditto.
(ftc_cache_resize, ftc_cache_init, FTC_Cache_Clear,
FTC_Cache_RemoveFaceID): Updated accordingly.