[cache] Revise the dynamic 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): Simplify the flow.
(ftc_cache_init): Stop over-allocating initially.
(FTC_Cache_Clear, FTC_Cache_RemoveFaceID): Updated accordingly.