Proposal 289 prevents SENDME-flooding by requiring the other side to
authenticate the data it has received.  But this data won't actually
be random if they are downloading a known resource.  "No problem",
we said, "let's fell the empty parts of our cells with some
randomness!" and we did that in #26871.

Unfortunately, if the relay data payloads are all completely full,
there won't be any empty parts for us to randomize.

Therefore, we now pick random "randomness windows" between
CIRCWINDOW_INCREMENT/2 and CIRCWINDOW_INCREMENT. We remember whether we have
sent a cell containing at least 16 bytes of randomness in that window.  If we
haven't, then when the window is exhausted, we send one.  (This window approach
is designed to lower the number of rng checks we have to do.  The number 16 is
pulled out of a hat to change the attacker's guessing difficulty to
"impossible".)

Implements 28646.