net/http: speed up ServeMux matching

Scanning through all path patterns is not necessary, since the
paths do not change frequently.  Instead, maintain a sorted list
of path prefixes and return the first match.

name            old time/op  new time/op  delta
ServerMatch-12   134ns ± 3%    17ns ± 4%  -86.95%  (p=0.000 n=19+20)

Change-Id: I15b4483dc30db413321435ee6815fc9bf2bcc546
Reviewed-on: https://go-review.googlesource.com/c/144937


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>