index: refactor snapshot iterator API

To make a memtx snapshot, we use the create_snapshot_iterator index
method. The method creates a 'frozen' iterator over an index - changes
done to the index after the iterator was created don't affect the
iterator output. Also, the iterator is safe to use from any thread.
This API works just fine for snapshots, but it's too limited to allow
creation of user read views so we need to rework it.

To make the existing snapshot infrastructure suitable for user read
views, this commit replaces the create_snapshot_iterator method with
create_read_view. The new method returns an index_read_view object,
which has the API similar to the read-only API of an index. A read view
object may only be created and destroyed in the tx thread, but it may be
used in any thread.

Currently, index_read_view has the only method - create_iterator, which
takes iterator type and key and returns an index_read_view_iterator
object. The iterator type and key arguments are ignored and we always
assume the iterator type to be ITER_ALL (asserted), but later on we will
fix this and also add a method to look up a tuple by key.

Closes #7194

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring