tree-index-opt: make struct key_def an independent member of struct space

Code review fixes for branch tree-index-opt:
to avoid unnecessary copying and settle the issue of
ownership of key_def structures, move their ownership
to struct space.

Now Index uses its key_def by pointer, and all key_defs are
allocated and freed separately in struct space
allocation/deallocation functions.

Fix coding style (use Lisp/Emacs style function
call style, as per LKML which we follow).