net/http: Optimize internal Cookie functions

- Pre-calculate *Cookie slice in read cookie functions
- readSetCookies: pre-allocs depending on the count of Set-Cookies, avoid
  allocs when no Set-Cookie is present
- readCookies: calculate *Cookie slice length
- 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()

Add BenchmarkCookieString()
1000000   2153 ns/op 520 B/op 10 allocs/op <= Original Go 1.7
1000000   1221 ns/op 384 B/op  3 allocs/op <= Modified Go 1.7