Parse the headers without splitting them upfront to reduce
memory allocations.

For non-pathological Cookie headers we can make
a good estimate of the number of cookies in there and preallocate
the slice of cookies

name              old time/op    new time/op    delta
CookieString-4      1.73µs ± 2%    1.70µs ± 5%     ~     (p=0.841 n=5+5)
ReadSetCookies-4    6.09µs ± 3%    5.93µs ± 3%     ~     (p=0.095 n=5+5)
ReadCookies-4       7.63µs ± 1%    6.41µs ± 4%  -15.99%  (p=0.008 n=5+5)

name              old alloc/op   new alloc/op   delta
CookieString-4        360B ± 0%      360B ± 0%     ~     (all equal)
ReadSetCookies-4      976B ± 0%      976B ± 0%     ~     (all equal)
ReadCookies-4       2.17kB ± 0%    1.84kB ± 0%  -15.13%  (p=0.008 n=5+5)

name              old allocs/op  new allocs/op  delta
CookieString-4        5.00 ± 0%      5.00 ± 0%     ~     (all equal)
ReadSetCookies-4      15.0 ± 0%      15.0 ± 0%     ~     (all equal)
ReadCookies-4         16.0 ± 0%      11.0 ± 0%  -31.25%  (p=0.008 n=5+5)

Change-Id: Ica1ca0d40c0d8d275134d1dfafb73f1082115826
Reviewed-on: https://go-review.googlesource.com/c/go/+/163617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>