Wake a link's thread only when it sleeps, once

Every queued record wrote the link's eventfd, a system call each, and woke
the thread to send a packet of a few records. A record queued now wakes
the thread only when it sleeps and nobody woke it yet; one queued while it
sends marks the link, and the thread looks again before it sleeps. With
records queued one at a time as fast as a thread can, 200,000 small ones
went at 400k a second, and now at 730k; queuing one costs 0.43 us, not
1.0.

research/experiments/019-idle-links/transport.c measures the transport
alone: a link and a listener in one process, no app, no store. A backlog
of 200,000 records of 61 bytes drains at 3 to 4 million a second, 0.24 us
a record on each side.

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