Serve an app with states from several connection threads

States have one owner, so a module with them ran on a single thread,
which spent most of its time in the kernel sending and receiving. With
--threads N such a module now gets N connection threads and one owner
thread: connection threads accept, parse and answer; the owner takes the
ready requests of every thread as one round, applies them in bursts with
one save each and hands each thread back its applied requests (eventfd),
which it completes and answers. A request's route and payload stay held
by its thread's runtime core until completion, so an accepted event runs
even when its connection closes first.

After a burst that saved, the owner gathers requests until none has come
for 100 µs (1 ms at most), so the requests its answers bring back share
the next save as they would on one thread: 30 events per save became 50
and the median latency of a durable post fell from 5.4 to 3.5 ms. A page
of 20 rows went from 87,600 to 141,900 answers a second on the Pi.

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