Pregrow buffer to reduce allocs on building cookie string.
We calc cookie name value and domain size (most distributed by value) and
add 110 extra characters allows to write most cookie attributes
without additional allocations in most cases.

name              old time/op    new time/op    delta
CookieString-4      1.65µs ± 3%    1.27µs ± 2%  -23.01%  (p=0.008 n=5+5)
ReadSetCookies-4    5.78µs ± 4%    5.67µs ± 3%     ~     (p=0.310 n=5+5)
ReadCookies-4       7.12µs ± 4%    7.04µs ± 3%     ~     (p=0.690 n=5+5)

name              old alloc/op   new alloc/op   delta
CookieString-4        360B ± 0%      144B ± 0%  -60.00%  (p=0.008 n=5+5)
ReadSetCookies-4      976B ± 0%      976B ± 0%     ~     (all equal)
ReadCookies-4       2.17kB ± 0%    2.17kB ± 0%     ~     (all equal)

name              old allocs/op  new allocs/op  delta
CookieString-4        5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)
ReadSetCookies-4      15.0 ± 0%      15.0 ± 0%     ~     (all equal)
ReadCookies-4         16.0 ± 0%      16.0 ± 0%     ~     (all equal)

Change-Id: I50defac954a135b785b3551342b00878429b3d7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/163017Reviewed-by: Volker Dobler <dr.volker.dobler@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>