time.now is somewhat expensive (much more expensive than nanotime),
in the common case when Time has monotonic time we don't actually
need to call time.now in Since/Until as we can do calculation
based purely on monotonic times.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 1%     ~     (p=0.151 n=5+5)
SetReadDeadline-6      261ns ± 0%   234ns ± 1%  -10.35%  (p=0.008 n=5+5)

Benchmark that only calls Until:

benchmark            old ns/op     new ns/op     delta
BenchmarkUntil       54.0          29.5          -45.37%

Update #25729

Change-Id: I5ac5af3eb1fe9f583cf79299f10b84501b1a0d7d
Reviewed-on: https://go-review.googlesource.com/c/146341
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>