memtx_tree: refactor key_def/cmp_def selector

 - Fold key_def/cmp_def selection logic (memtx_tree_index_cmp_def).
   There are only a couple places that use it so it isn't going to
   hurt readability.
 - Add memtx_tree_cmp_def helper that returns the definition currently
   used by a memtx tree. In contrast to removed memtx_tree_index_cmp_def
   this one doesn't check index_def. Instead it returns memtx_tree arg.
   We will use it to compute key/tuple hints.