Unnecessary buf_pool.page_hash lookups

In the function btr_blob_free() there is an unnecessary lookup of buf_pool.page_hash. If we acquired an additional buffer-fix on the page before mtr_t::commit() and released it after acquiring buf_pool.mutex, we would know that the page still exists in the buffer pool and can be attempted to be freed, without having to look it up in buf_pool.page_hash again.

There could be similar suboptimal patterns elsewhere as well.