SHOW ENGINE INNODB MUTEX functionality is completely removed,
as are the InnoDB latching order checks.

We will enforce innodb_fatal_semaphore_wait_threshold
only for dict_sys.mutex and lock_sys.mutex.

dict_sys_t::mutex_lock(): A single entry point for dict_sys.mutex.

lock_sys_t::mutex_lock(): A single entry point for lock_sys.mutex.

FIXME: srv_sys should be removed altogether; it is duplicating tpool
functionality.

fil_crypt_threads_init(): To prevent SAFE_MUTEX warnings, we must
not hold fil_system.mutex.

fil_close_all_files(): To prevent SAFE_MUTEX warnings for
fil_space_destroy_crypt_data(), we must not hold fil_system.mutex
while invoking fil_space_free_low() on a detached tablespace.