Local perf test

This is on my laptop (4 core, 8 GB). Go 1.8.

Using the just commited utils in the "local-perf" directory.

~~~
(pprof) top10
0.09s of 7.06s total ( 1.27%)
Dropped 108 nodes (cum <= 0.04s)
Showing top 10 nodes out of 165 (cum >= 1.53s)
flat flat% sum% cum cum%
0 0% 0% 5.24s 74.22% runtime.goexit
0 0% 0% 2.95s 41.78% github.com/miekg/dns.(*Server).serve
0 0% 0% 2.46s 34.84% github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket.func1
0 0% 0% 2.46s 34.84% github.com/miekg/dns.HandlerFunc.ServeDNS
0.04s 0.57% 0.57% 2.45s 34.70% github.com/coredns/coredns/core/dnsserver.(*Server).ServeDNS
0 0% 0.57% 2.33s 33.00% github.com/coredns/coredns/middleware/whoami.(*Whoami).ServeDNS
0.04s 0.57% 1.13% 2.33s 33.00% github.com/coredns/coredns/middleware/whoami.Whoami.ServeDNS
0.01s 0.14% 1.27% 1.56s 22.10% github.com/miekg/dns.(*response).WriteMsg
0 0% 1.27% 1.53s 21.67% github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket
0 0% 1.27% 1.53s 21.67% github.com/mholt/caddy.startServers.func1
~~~
Corefile included in that repo. CoreDNS is running with:
`./coredns -conf ~/git/coredns/perf-tests/local-perf/Corefile`

profile attached.
[pprof.coredns.localhost:6053.samples.cpu.001.pb.gz](https://github.com/coredns/perf-tests/files/966817/pprof.coredns.localhost.6053.samples.cpu.001.pb.gz)