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/dnssec/cache_test.go

@@ -4,10 +4,9 @@ import (
 	"testing"
 	"time"
 
+	"github.com/coredns/coredns/middleware/pkg/cache"
 	"github.com/coredns/coredns/middleware/test"
 	"github.com/coredns/coredns/request"
-
-	"github.com/hashicorp/golang-lru"
 )
 
 func TestCacheSet(t *testing.T) {
@@ -21,11 +20,11 @@ func TestCacheSet(t *testing.T) {
 		t.Fatalf("failed to parse key: %v\n", err)
 	}
 
-	cache, _ := lru.New(defaultCap)
+	c := cache.New(defaultCap)
 	m := testMsg()
 	state := request.Request{Req: m}
-	k := key(m.Answer) // calculate *before* we add the sig
-	d := New([]string{"miek.nl."}, []*DNSKEY{dnskey}, nil, cache)
+	k := hash(m.Answer) // calculate *before* we add the sig
+	d := New([]string{"miek.nl."}, []*DNSKEY{dnskey}, nil, c)
 	m = d.Sign(state, "miek.nl.", time.Now().UTC())
 
 	_, ok := d.get(k)