arpa: limit number of idle notifications sent per tick

The ARP agent can send idle notifications much faster than OFConnectionManager
can write them to the socket. Limiting the number of messages sent per tick
gives OFConnectionManager a chance to send them in its socket write callback.
arpa: check timers more frequently when running behind

Now that there's a limit on the number of flows than can be expired per timer
wakeup, we'd be limited to 32 idle notifications per second with our 1 Hz
timer. Instead, increase the timer frequency to 10 Hz when the load is high.
Merge into master from pull request #57:
throttle ARP idle notifications (https://github.com/floodlight/switchlight-common/pull/57)