Measure the CPU a record costs a link, against quinn (096)

M5 of design 011. At first a link spent eight times quinn's CPU on a small
record: scans of the backlog, a system call and allocations a record, 19
bytes of framing. After the seven changes before this one, the transport
alone (transport.c; a backlog drained, every record checked) spends per
record 0.30 us for 61 bytes (quinn 0.27: 1.06 to 1.12 times over three
campaigns, at the design's 10 % line), 2.91 us for 1,013 bytes (quinn
3.88) and 45.2 us for 16,013 bytes (quinn 60.9): 25 % less for the larger
ones, and 30 % more records a second. What is left for small records is
per packet: the stream's names and the RECORDS head. quinn's numbers carry
no confirmation of storing and no exactly once.

Between two pure-serve nodes the links spend more per record, woken at
the receiving app's pace. Filling the sender showed the outbox is written
whole at every save, as the numbers heard were (094): the next thing to
change.

run.py gains --transport and --drain; the quinn service reads a drain a
chunk at a time and its client writes 64 KiB at a time.

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