Prop #254: Clarify special cases for bin 0 and inf bin-1.

The intermediate bins thus divide the timespan range_usec 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_usec/2, CIRCPAD_DELAY_INFINITE).

thus covers [start_usec+rtt_estimate+range_usec/2,

start_usec+rtt_estimate+range_usec).

This exponentially increasing bin width allows the histograms to most

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

When all bins are empty in a histogram, the padding machine emits the internal

"bins empty" event to itself.

Bin 0 and the bin before the infinity bin both have special rules for purposes

of token removal. While removing tokens, all values less than bin 0 are

treated as part of bin 0, and all values greater than

start_usec+rtt_estimate+range_sec are treated as part of the bin before the

infinity bin.

3.2. State Machine Selection

Clients will select which of the defined available padding machines to use