Sometimes the desired ancillary message does not come head-first.

If there are 16 slots, then the maximum index is 15.

This simplifies the code a lot, since it gives us the notion of a session
for free.  And congestion control, of course.

This is not quite ready yet: we don't wait for the peer's handshake before
initiating flooding, and we don't perform duplicate suppression.  Doing that
will require a different data structure.