net/http: add new Server benchmark
net/http: add new Server benchmark

The new BenchmarkServer avoids profiling the client code
by running it in a child process.

R=rsc
CC=golang-dev
http://codereview.appspot.com/6260053

gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme