MDEV-21544 instrument sync_array waits with tpool::wait_begin/end

threadpool has to know about long waits, to be able to create or wakeup
new threads, or it can become slow, if it just relies on maintainence
timer.

storage/innobase/sync/sync0arr.cc

@@ -468,7 +468,9 @@ sync_array_wait_event(
 #endif /* UNIV_DEBUG */
 	sync_array_exit(arr);
 
+	tpool::tpool_wait_begin();
 	os_event_wait_low(sync_cell_get_event(cell), cell->signal_count);
+	tpool::tpool_wait_end();
 
 	sync_array_free_cell(arr, cell);