Usleep(100) in runqgrab negatively affects latency and throughput
of parallel application. We are sleeping instead of doing useful work.
This is effect is particularly visible on windows where minimal
sleep duration is 1-15ms.

Reduce sleep from 100us to 3us and use osyield on windows.
Sync chan send/recv takes ~50ns, so 3us gives us ~50x overshoot.

benchmark                    old ns/op     new ns/op     delta
BenchmarkChanSync-12         216           217           +0.46%
BenchmarkChanSyncWork-12     27213         25816         -5.13%

CPU consumption goes up from 106% to 108% in the first case,
and from 107% to 125% in the second case.

Test case from #14790 on windows:

BenchmarkDefaultResolution-8  4583372   29720    -99.35%
Benchmark1ms-8                992056    30701    -96.91%

99-th latency percentile for HTTP request serving is improved by up to 15%
(see http://golang.org/cl/20835 for details).

The following benchmarks are from the change that originally added this sleep
(see https://golang.org/s/go15gomaxprocs):

name        old time/op  new time/op  delta
Chain       22.6µs ± 2%  22.7µs ± 6%    ~      (p=0.905 n=9+10)
ChainBuf    22.4µs ± 3%  22.5µs ± 4%    ~      (p=0.780 n=9+10)
Chain-2     23.5µs ± 4%  24.9µs ± 1%  +5.66%   (p=0.000 n=10+9)
ChainBuf-2  23.7µs ± 1%  24.4µs ± 1%  +3.31%   (p=0.000 n=9+10)
Chain-4     24.2µs ± 2%  25.1µs ± 3%  +3.70%   (p=0.000 n=9+10)
ChainBuf-4  24.4µs ± 5%  25.0µs ± 2%  +2.37%  (p=0.023 n=10+10)
Powser       2.37s ± 1%   2.37s ± 1%    ~       (p=0.423 n=8+9)
Powser-2     2.48s ± 2%   2.57s ± 2%  +3.74%   (p=0.000 n=10+9)
Powser-4     2.66s ± 1%   2.75s ± 1%  +3.40%  (p=0.000 n=10+10)
Sieve        13.3s ± 2%   13.3s ± 2%    ~      (p=1.000 n=10+9)
Sieve-2      7.00s ± 2%   7.44s ±16%    ~      (p=0.408 n=8+10)
Sieve-4      4.13s ±21%   3.85s ±22%    ~       (p=0.113 n=9+9)

Fixes #14790

Change-Id: Ie7c6a1c4f9c8eb2f5d65ab127a3845386d6f8b5d
Reviewed-on: https://go-review.googlesource.com/20835Reviewed-by: Austin Clements <austin@google.com>