box: replace CTID_STRUCT_ITERATOR_REF with CTID_STRUCT_ITERATOR_PTR

In Lua C code 'struct iterator&' and 'struct iterator*' are handled in
exactly the same way. However, if we create an iterator object in Lua by
calling box_index_iterator via FFI, and pass the object to Lua C, it
will have 'struct iterator*' type, not 'struct iterator&'. In the next
patch we need Lua C functions to work with such objects so let's replace
'struct iterator&' with 'struct iterator*' in C code.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring