vinyl: remove vy_run->page_cache

* Replace vy_run->page_cache with LRU cache in vy_run_iterator
* Remove vy_run_iterator_lock_page()/vy_run_iterator_unlock_page()
* Refactor struct vy_page to remove dangling pointers to vy_page_info
* Fix error handling in vy_run_iterator and vy_merge_iterator
* Remove special optimization for case when key == page->min_key