Answer a round's reads before its writes on the owner

With durable writes going on, a read waited for the save of the burst it
landed in. A round's requests come from different connections, one
request each at a time, so nothing orders them: the owner now applies a
round's reads first and answers them at once, seeing only saved states,
then its writes in bursts. 8 writers and 56 readers of the scoreboard,
owner mode: reads 7,100-7,700 to 16,100-16,600 a second, their p99 55 to
13 ms, writes 1,000-1,100 to 2,300 a second. A single thread keeps the
order: it waits out each save itself, and reads answered first come back
first and split its next writes over more saves (the owner gathers after
a save instead); there reads are 12,800-13,400 a second as before.

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