New cache implementation and prefetch handing in mw/cache (#731)

* cache: add sharded cache implementation

Add Cache impl and a few tests. This cache is 256-way sharded, mainly
so each shard has it's own lock. The main cache structure is a readonly
jump plane into the right shard.

This should remove the single lock contention on the main lock and
provide more concurrent throughput - Obviously this hasn't been tested
or measured.

The key into the cache was made a uint32 (hash.fnv) and the hashing op
is not using strings.ToLower anymore remove any GC in that code path.

* here too

* Minimum shard size

* typos

* blurp

* small cleanups no defer

* typo

* Add freq based on Johns idea

* cherry-pick conflict resolv

* typo

* update from early code review from john

* add prefetch to the cache

* mw/cache: add prefetch

* remove println

* remove comment

* Fix tests

* Test prefetch in setup

* Add start of cache

* try add diff cache options

* Add hacky testcase

* not needed

* allow the use of a percentage for prefetch

If the TTL falls below xx% do a prefetch, if the record was popular.
Some other fixes and correctly prefetch only popular records.

middleware/cache/prefech_test.go

@@ -0,0 +1,54 @@
+package cache
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/middleware"
+	"github.com/coredns/coredns/middleware/pkg/cache"
+	"github.com/coredns/coredns/middleware/pkg/dnsrecorder"
+
+	"github.com/coredns/coredns/middleware/test"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+var p = false
+
+func TestPrefetch(t *testing.T) {
+	c := &Cache{Zones: []string{"."}, pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxTTL}
+	c.pcache = cache.New(c.pcap)
+	c.ncache = cache.New(c.ncap)
+	c.prefetch = 1
+	c.duration = 1 * time.Second
+	c.Next = PrefetchHandler(t, dns.RcodeSuccess, nil)
+
+	ctx := context.TODO()
+
+	req := new(dns.Msg)
+	req.SetQuestion("lowttl.example.org.", dns.TypeA)
+
+	rec := dnsrecorder.New(&test.ResponseWriter{})
+
+	c.ServeDNS(ctx, rec, req)
+	p = true // prefetch should be true for the 2nd fetch
+	c.ServeDNS(ctx, rec, req)
+}
+
+func PrefetchHandler(t *testing.T, rcode int, err error) middleware.Handler {
+	return middleware.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+		m := new(dns.Msg)
+		m.SetQuestion("lowttl.example.org.", dns.TypeA)
+		m.Response = true
+		m.RecursionAvailable = true
+		m.Answer = append(m.Answer, test.A("lowttl.example.org. 80 IN A 127.0.0.53"))
+		if p != w.(*ResponseWriter).prefetch {
+			err = fmt.Errorf("cache prefetch not equal to p: got %t, want %t", p, w.(*ResponseWriter).prefetch)
+			t.Fatal(err)
+		}
+
+		w.WriteMsg(m)
+		return rcode, err
+	})
+}