buf_block_t::check_index_page_at_flush: Note that this field is not
protected by any mutex. Make it a separate field, not a bitfield that
could share the machine word with other fields.

buf_block_t::lock_hash_val: Note that this field is protected by
buf_block_t::lock (or during block creation, by buf_pool_mutex and
buf_block_t::mutex).

buf_block_get_lock_hash_val(): Assert that block->lock is held by the
current thread.

Issue #465, rb://267 approved by Inaam Rana