net/http: optimize internal cookie functions

net/http: optimize internal cookie functions

- precalculate *Cookie slice in read cookie functions
- readSetCookies: pre-allocs depending on the count of Set-Cookies
- rename success variable to ok; avoid else
- refactor Cookie.String to use less allocations
- remove fmt package and replace with writes to a bytes.Buffer
- add BenchmarkReadSetCookies and BenchmarkReadCookies

name              old time/op    new time/op    delta
CookieString-8      1.42µs ± 2%    0.78µs ± 1%  -45.36%        (p=0.000 n=10+10)
ReadSetCookies-8    3.46µs ± 1%    3.42µs ± 2%   -1.39%        (p=0.001 n=10+10)
ReadCookies-8       5.12µs ± 1%    5.15µs ± 2%     ~           (p=0.393 n=10+10)

name              old alloc/op   new alloc/op   delta
CookieString-8        520B ± 0%      384B ± 0%  -26.15%        (p=0.000 n=10+10)
ReadSetCookies-8      968B ± 0%      960B ± 0%   -0.83%        (p=0.000 n=10+10)
ReadCookies-8       2.01kB ± 0%    2.01kB ± 0%     ~     (all samples are equal)

name              old allocs/op  new allocs/op  delta
CookieString-8        10.0 ± 0%       3.0 ± 0%  -70.00%        (p=0.000 n=10+10)
ReadSetCookies-8      18.0 ± 0%      17.0 ± 0%   -5.56%        (p=0.000 n=10+10)
ReadCookies-8         16.0 ± 0%      16.0 ± 0%     ~     (all samples are equal)

Change-Id: I870670987f10f3e52f9c657cfb8e6eaaa97a6162
Reviewed-on: https://go-review.googlesource.com/27850
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>