The DefaultServeMux included in net/http uses a map to store routes,
but iterates all keys for every request to allow longer paths.

This change checks the map for an exact match first.

To check performance was better, BenchmarkServeMux has been added -
this adds >100 routes and checks the matches.

Exact matches are faster and more predictable on this benchmark
and on most existing package benchmarks.

https://perf.golang.org/search?q=upload:20170312.1

ServeMux-4  2.02ms ± 2%	0.04ms ± 2%  −98.08%  (p=0.004 n=5+6)

https://perf.golang.org/search?q=upload:20170312.2

ReadRequestChrome-4	184MB/s  ± 8%	186MB/s  ± 1%	~
ReadRequestCurl-4	45.0MB/s ± 1%	46.2MB/s ± 1%	+2.71%
Read...Apachebench-4	45.8MB/s ±13%	48.7MB/s ± 1%	~
ReadRequestSiege-4	63.6MB/s ± 5%	69.2MB/s ± 1%	+8.75%
ReadRequestWrk-4	30.9MB/s ± 9%	34.4MB/s ± 2%	+11.25%

Change-Id: I8afafcb956f07197419d545a9f1c03ecaa307384
Reviewed-on: https://go-review.googlesource.com/38057Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>