Any modification of buf_pool.page_hash is supposed to be protected
by buf_pool.mutex and page_hash_latch::write_lock(). The buffer pool
watch mechanism of the InnoDB change buffer was violating that
ever since commit b1ab211d (MDEV-15053).

buf_pool_t::watch_set(): Extend the critical section of buf_pool.mutex.

buf_pool_t::watch_unset(): Define non-inline, because
calls are infrequent and this function became larger.
If we have to detach a sentinel from page_hash,
do it while holding both the mutex and the exclusive hash latch.

buf_pool_t::watch_remove(): Assert that the mutex is being held.

buf_page_init_for_read(): Remove some work-arounds for
previously encountered race conditions related to buf_pool.watch.