thread: avoid repeated polls while scanning available threads

Poll for external events just once before starting a search for an
available thread, instead of polling every iteration in the search.

racket/src/thread/schedule.rkt

@@ -55,22 +55,22 @@
   (set! thread-swap-count 0))
  
 (define (select-thread! [pending-callbacks null])
+  (define callbacks (if (null? pending-callbacks)
+                        (host:poll-async-callbacks)
+                        pending-callbacks))
+  (host:poll-will-executors)
+  (poll-custodian-will-executor)
+  (check-external-events 'fast)
+  (call-pre-poll-external-callbacks)
+  (check-place-activity)
+  (check-queued-custodian-shutdown)
+  (when (and (null? callbacks)
+             (all-threads-poll-done?)
+             (waiting-on-external-or-idle?))
+    (or (check-external-events 'slow)
+        (try-post-idle)
+        (process-sleep)))
   (let loop ([g root-thread-group] [pending-callbacks pending-callbacks] [none-k maybe-done])
-    (define callbacks (if (null? pending-callbacks)
-                          (host:poll-async-callbacks)
-                          pending-callbacks))
-    (host:poll-will-executors)
-    (poll-custodian-will-executor)
-    (check-external-events 'fast)
-    (call-pre-poll-external-callbacks)
-    (check-place-activity)
-    (check-queued-custodian-shutdown)
-    (when (and (null? callbacks)
-               (all-threads-poll-done?)
-               (waiting-on-external-or-idle?))
-      (or (check-external-events 'slow)
-          (try-post-idle)
-          (process-sleep)))
     (define child (thread-group-next! g))
     (cond
       [(not child) (none-k callbacks)]