box: introduce result processor

A convenient helper that is supposed to apply various transformations
to a tuple fetched from a space before returning it to the user.
Usage:

  struct result_processor res_proc;
  result_process_prepare(&res_proc, space);
  rc = index_get(index, key, part_count, result);
  result_process(&res_proc, &rc, result);

We need to split the procedure into two parts, because index_get and
similar methods may yield in case of Vinyl. The 'prepare' method is
supposed to pin all data structures needed to process the result in the
result_processor struct while the 'process' method is supposed to
release them.

Currently, the new methods do absolutely nothing. They will be used to
convert tuples during space upgrade.

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal