box: initialize index_def::cmp_def in index_def constructor

We're going to need a key def of secondary key augmented
with key parts from the primary key in memtx engine as well.
Initialize it when creating struct index_def.

This patch reverts f42e1c15,
i.e. the fix for gh-2386 (box_index_key_def() as part of module
API). Exporting this function to C API was premature. Luckily
it is not used anywhere except 1.8. For 1.8 this patch introduces
a faster function, space_index_key_def(), which takes a pointer
to space rather than space id.