Drain a link's backlog without scanning it: 370k to 990k records a second

A backlog made the link slow in its square: the sender looked at every
queued record's pieces twice a turn to know whether any waited to be
stored, and the receiver moved all its held records down each time it let
the first one go.

- The sender counts the pieces received and not stored, and keeps where
  the unsent ones begin; every change of a piece's state goes through one
  place that keeps both.
- The receiver keeps its held records from a head index. One in order goes
  last, and one out of order finds its place by halving. The record's bytes
  and its pieces' marks are one allocation.
- Packets go in bursts with UDP GSO, 54 at a time: 0.45 us a packet on this
  Pi, against 2.4 us for a send each. A full packet is padded to a whole
  datagram (PADDING frames, as QUIC does). Datagrams are taken 64 at a time
  (recvmmsg).

Twenty thousand saved sends of 61 bytes drained at 370k a second; now at
990k. Per record, the sender's link thread took 1.1 us and now takes 0.5;
the receiver's took 2.4 and now takes 0.5.

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