"free" innodb pages from dropped table/index/(deleted?)/table rebuild

From previous, Evict all pages that became useless after DROP TABLE or a table rebuild.

Currently they exist in the innodb buffer pool and will be evicted once their LRU drops as they won't exist again.

To be memory efficient we should `set_os_unused()` to allow the OS to reclaim these memory if required.

I don't think we want to do the synchronously otherwise the single query performance drops. So maybe this is some additional thing that happens in the purge thread.