Prop 254: Describe token generation more clearly

The first bin of the histogram (bin 0) has 0 width, with a delay value of

start_usec+rtt_estimate (from the machine definition, and rtt estimate above).

The bin before the "infinity bin" has a time value of

start_usec+rtt_estimate+range_sec*USEC_PER_SEC.

The bins between these two points are exponentially spaced, so that smaller

bin indexes represent narrower time ranges, doubling up until the last bin

range of [(start_usec+rtt_estimate+range_sec*USEC_PER_SEC)/2,

start_usec+rtt_estimate+range_sec*USEC_PER_SEC).

The remaining bins are exponentially spaced, starting at this offset and

covering the range of the histogram, which is range_sec*USEC_PER_SEC.

The intermediate bins thus divide the timespan range_sec*USEC_PER_SEC with

offset start_usec+rtt_estimate, so that smaller bin indexes represent narrower

time ranges, doubling up until the last bin. The last bin before the "infinity

bin" thus covers [start_usec+rtt_estimate+range_sec*USEC_PER_SEC/2,

CIRCPAD_DELAY_INFINITE).

This exponentially increasing bin width allows the histograms to most

accurately represent small interpacket delay (where accuracy is needed), and