Continue porting feature-index-2objc branch.

Use the index iterator API in memcached.m,
instead of hard-coded access to index internals,
which broke index abstraction.

This (temporarily) can (somewhat) break expire
in presense of SELECT command against memcached namespace,
since SELECT uses the same iterator.

Hopefully, it's only temporary, since we'll soon
be able to have more than one independent generic iterator
over the same index.