Sleep between deferred requests on paused handler

It looks like a launcher can busy loop and starve other threads from
running when a provider is at quota and a lot of requests are deferred.

Sleep 250ms between deferred requests to not hog the CPU.

Change-Id: I3dbf74ee2fe50308d1b5b286f091a052ec6c5ef9

nodepool/launcher.py

@@ -238,6 +238,7 @@ class PoolWorker(threading.Thread, stats.StatsReporter):
 
             if self.paused_handlers and not reasons_to_decline:
                 self.log.debug("Handler is paused, deferring request")
+                time.sleep(0.25)
                 continue
 
             # At this point, we are either unpaused, or we know we