Perf improvements in HTTP protocol

[slinkydeveloper]

Because net/http.Header states that headers are keyed by their canonical mime header representation (https://golang.org/pkg/net/http/#Header), I optimized on avoiding any middle allocations for accessing the attributes map and i pre-compute the header names for the various attributes.

I've also rewrote the computation of the extension key, in order to allocate only one new string.

Results

benchmark                                   old ns/op     new ns/op     delta
BenchmarkHttpWithToEvent-8                  17220         15179         -11.85%
BenchmarkNoDataHttpWithToEvent-8            16575         14101         -14.93%
BenchmarkHttpWithBuffering-8                10642         8703          -18.22%
BenchmarkHttpWithDirect-8                   9080          7613          -16.16%
BenchmarkNoDataHttpWithBuffering-8          10041         8613          -14.22%
BenchmarkNoDataHttpWithDirect-8             9225          7785          -15.61%

benchmark                                   old allocs     new allocs     delta
BenchmarkHttpWithToEvent-8                  136            108            -20.59%
BenchmarkNoDataHttpWithToEvent-8            130            102            -21.54%
BenchmarkHttpWithBuffering-8                108            80             -25.93%
BenchmarkHttpWithDirect-8                   101            73             -27.72%
BenchmarkNoDataHttpWithBuffering-8          106            78             -26.42%
BenchmarkNoDataHttpWithDirect-8             99             71             -28.28%

benchmark                                   old bytes     new bytes     delta
BenchmarkHttpWithToEvent-8                  8385          8061          -3.86%
BenchmarkNoDataHttpWithToEvent-8            7759          7430          -4.24%
BenchmarkHttpWithBuffering-8                5816          5498          -5.47%
BenchmarkHttpWithDirect-8                   4502          4173          -7.31%
BenchmarkNoDataHttpWithBuffering-8          5777          5460          -5.49%
BenchmarkNoDataHttpWithDirect-8             4467          4151          -7.07%

[markusthoemmes]

Microoptimizations are fun! I like :)

[n3wscott]

This is only true for binary mode.

[slinkydeveloper]

I know that, Version and related interfaces are used only for binary messages

[n3wscott]

LGTM