memtx: introduce createSnapshotIterator method in index

Memtx checkpointing subsystem uses createReadViewForIterator and
destroyReadViewForIterator methods of Index. So these methods
are used only for ALL iterators before actual iteration starts.

 * no other cases of these methods using are covered by tests,
for example EQ iteration or using in the middle of iteration.

 * creation of general read view iterator is an overkill for
such a simple case as ALL iterator.

 * memtx read view iterators are incompatible with lazy lookup
and restoration patterns that are necessary for implementing
stable iterators.

Introduce new createSnapshotIterator method that would create
a special iterator for checkpointing and remove misleading
createReadViewForIterator and destroyReadViewForIterator methods.