vinyl: refactor vy_read_iterator_next

The function looks rather messy now. Let's rearrange its code to make it
look better and facilitate further development:

 - Move the debug checks making sure that the next key meets search
   criteria and doesn't violate the iteration order to the place where
   we look up the next key, i.e. to vy_read_iterator_next_key.

 - Move result tracking in tx read set from vy_read_iterator_next_key
   to vy_read_iterator_next so that the former now just advances the
   iterator to the next key and does nothing else.

 - Remove vy_read_iterator::search_started. We don't really need it as
   we can use (vy_read_iterator::last_stmt == NULL) condition instead.

 - Rearrange the code of vy_read_iterator_next to reduce indentation
   level.

 - Remove/rename some variables and labels. Add some comments.