add wait_available() to Pool and GreenletSet

gevent/pool.py

@@ -181,6 +181,9 @@ class GreenletSet(object):
     def full(self):
         return False
 
+    def wait_available(self):
+        pass
+
 
 class Pool(GreenletSet):
 
@@ -194,6 +197,9 @@ class Pool(GreenletSet):
         self._available_event = Event()
         self._available_event.set()
 
+    def wait_available(self):
+        self._available_event.wait()
+
     def full(self):
         return self.free_count() <= 0