Standard net/http is known to be putting more focus on idiomaticy instead
of performance. Use github.com/valyala/fasthttp whose main focus is
performance. It is still all pure go, but it is different Go code.

On my laptop the result for / is:

(-fast=0):

	kirr@deco:~/src/tools/net/wrk$ ./wrk -t 4 -c 40 -d 20 http://127.0.0.1:8080
	Running 20s test @ http://127.0.0.1:8080
	  4 threads and 40 connections
	  Thread Stats   Avg      Stdev     Max   +/- Stdev
	    Latency     1.02ms    2.58ms  63.90ms   93.04%
	    Req/Sec    27.33k     3.98k   39.64k    68.38%
	  2184698 requests in 20.09s, 270.85MB read
	Requests/sec: 108752.84
	Transfer/sec:     13.48MB

(-fast=1):

	kirr@deco:~/src/tools/net/wrk$ ./wrk -t 4 -c 40 -d 20 http://127.0.0.1:8080
	Running 20s test @ http://127.0.0.1:8080
	  4 threads and 40 connections
	  Thread Stats   Avg      Stdev     Max   +/- Stdev
	    Latency   284.88us  645.44us  20.31ms   96.55%
	    Req/Sec    48.97k     4.16k   71.85k    76.75%
	  3900962 requests in 20.02s, 550.60MB read
	Requests/sec: 194809.91
	Transfer/sec:     27.50MB

i.e. it is ~ 1.8x improvement.

I tested this only briefly and had not looked in detail what
improvements could also be there.

/cc @bminusl, @jp

/reviewed-on !1