arpa: check timers more frequently when running behind
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.

floodlight/switchlight-common,floodlight/switchlight-common,floodlight/switchlight-common