avoid loop without yield when sockets are >60 (#138)

For Python 2.x, sleep_interval is 0 if len(self.sockets) is above
self.ping_timeout (default: 60). With eventlet, self.sleep(0) looks it
does not yield to the event loop, queueing work which is not processed.
In this condition, the symptom is the server using 100% of CPU after
some time, even when count of sockets drop below self.sockets.

This fix forces sleep_interval to be a float, not equal to 0, forcing
eventlet to yield, processing the queued work and avoiding CPU to be at
100%.

This fix does nothing for Python 3.x.

engineio/server.py

@@ -652,7 +652,7 @@ def _service_task(self):  # pragma: no cover
                 continue
 
             # go through the entire client list in a ping interval cycle
-            sleep_interval = self.ping_timeout / len(self.sockets)
+            sleep_interval = float(self.ping_timeout) / len(self.sockets)
 
             try:
                 # iterate over the current clients