context: remove one allocation in timerCtx

context: remove one allocation in timerCtx

Embed the cancelCtx into timerCtx, instead of allocating both separately.

name                               old time/op    new time/op    delta
WithTimeout/concurrency=40-16        2.21µs ±11%    2.08µs ± 5%   -5.76%  (p=0.011 n=10+10)
WithTimeout/concurrency=4000-16      1.94µs ± 6%    1.86µs ±10%     ~     (p=0.099 n=9+10)
WithTimeout/concurrency=400000-16    1.86µs ± 7%    1.83µs ±10%     ~     (p=0.353 n=10+10)

name                               old alloc/op   new alloc/op   delta
WithTimeout/concurrency=40-16        2.56kB ± 0%    2.40kB ± 0%   -6.25%  (p=0.001 n=8+9)
WithTimeout/concurrency=4000-16      2.56kB ± 0%    2.40kB ± 0%   -6.25%  (p=0.000 n=9+10)
WithTimeout/concurrency=400000-16    2.56kB ± 0%    2.40kB ± 0%   -6.26%  (p=0.000 n=9+9)

name                               old allocs/op  new allocs/op  delta
WithTimeout/concurrency=40-16          50.0 ± 0%      40.0 ± 0%  -20.00%  (p=0.000 n=10+10)
WithTimeout/concurrency=4000-16        50.0 ± 0%      40.0 ± 0%  -20.00%  (p=0.000 n=10+10)
WithTimeout/concurrency=400000-16      50.0 ± 0%      40.0 ± 0%  -20.00%  (p=0.000 n=10+10)

Change-Id: Ia0460db3b8412fbaa6f1539fd6b4fb1b873896c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/463999
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>