key_def: cleanup virtual function initialization

 - Rename key_def_set_cmp to key_def_set_func, because it sets not only
   comparators these days.
 - Rename tuple_hash_func_set and tuple_extract_key_set to
   key_def_set_hash_func and key_def_set_extract_func, because it's more
   like subject-verb-object naming convention used throughout the code.
 - Introduce key_def_set_compare_func and use it instead of setting
   comparators explicitly in key_def.c.