vinyl: cache iterator: consolidate curr_stmt updates

Currently, vy_cache_iterator->curr_stmt is updated by top-level iterator
functions - next, skip, restore - which results in code duplication and
spreads core logic among multiple places. To reduce the amount of code
and make it generally easier to follow, this patch moves the updates to
low level functions - step, seek. It also makes the seek method return
the stop flag, which makes it similar to step, thus making the code more
consistent.