#12701.

Synced from MySQL.

Make comments about buf_block_t::index more accurate.

Note that buf_block_t::index should be protected by btr_search_latch
or an s-latch or x-latch on the index page.
btr_search_drop_page_hash_index(): Read block->index while holding
btr_search_latch and use the cached value in the loop.  Remove some
redundant assertions.

Mostly fix bug #13778, when FOREIGN_KEY_CHECKS=0 we still need to check
that datatypes between foreign key references are compatible.

Add test cases (also for bug #9802).

to avoid warnings.

Adapt callers.