Read the clock once a round

A connection's deadline was counted from a fresh clock_gettime on every
read and write, three times a request, and every event took the wall
clock again: 5.7% of a small read's user time. A connection thread now
reads the monotonic clock once after each epoll_wait and deadlines count
from that (they are 10 s by default and swept every second), and a
round's events share the time it began.

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