memtx: fix HASH index 'GT' iterator `next` method set incorrectly

The `next` method of memtx HASH index 'GT' iterator is initially set to
'GT' and is supposed to be set to 'GE' after first iteration: it is
mistakenly set to the 'base' method instead of the full method which also
does tuple clarification — this allows dirty reads. Move the `next` method
change on first iteration to `WRAP_ITERATOR_METHOD` for clarity and
correctness.

Closes #7477

NO_DOC=bugfix