MDEV-23855: Shrink fil_space_t

Merge n_pending_ios, n_pending_ops to std::atomic<uint32_t> n_pending.
Change some more fil_space_t members to uint32_t to reduce
the memory footprint.

fil_space_t::add(), fil_ibd_create(): Attach the already opened
handle to the tablespace, and enforce the fil_system.n_open limit.

dict_boot(): Initialize fil_system.max_assigned_id.

srv_boot(): Call srv_thread_pool_init() before anything else,
so that files should be opened in the correct mode on Windows.

fil_ibd_create(): Create the file in OS_FILE_AIO mode, just like
fil_node_open_file_low() does it.

dict_table_t::is_accessible(): Replaces fil_table_accessible().

Reviewed by: Vladislav Vaintroub

storage/innobase/include/fsp0fsp.h

@@ -466,9 +466,9 @@ free pages available.
 @return true if we were able to make the reservation */
 bool
 fsp_reserve_free_extents(
-	ulint*		n_reserved,
+	uint32_t*	n_reserved,
 	fil_space_t*	space,
-	ulint		n_ext,
+	uint32_t	n_ext,
 	fsp_reserve_t	alloc_type,
 	mtr_t*		mtr,
 	uint32_t	n_pages = 2);