x/net/http2: split cookie header as recommended by spec

[rs]

As per 8.1.2.5, To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs.

After some tests, it does positively impact compression rate and reduces the eviction rate on the dynamic table (flavor may vary depending traffic patterns).

[gopherbot]

Change https://golang.org/cl/155657 mentions this issue: x/net/http2: split cookie pair into separate hpack header fields

[katiehockman]

/cc @bradfitz

[bradfitz]

SGTM.