Reduce default burst limit to rate limit / 4.

Reduce default burst limit to rate limit / 4.

With the previous default burst limit of rate limit * 2, we would queue
up 2 seconds worth of packet_in messages.  This is not only much more
than actually needed, it causes an actual problem: the datapath only
retains buffered packets for up to 1 second, by default, so that flow
setups sent in response have no packet to work with.