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/setup.go

@@ -6,8 +6,8 @@ import (
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/middleware"
+	"github.com/coredns/coredns/middleware/pkg/cache"
 
-	"github.com/hashicorp/golang-lru"
 	"github.com/mholt/caddy"
 )
 
@@ -24,12 +24,9 @@ func setup(c *caddy.Controller) error {
 		return middleware.Error("dnssec", err)
 	}
 
-	cache, err := lru.New(capacity)
-	if err != nil {
-		return err
-	}
+	ca := cache.New(capacity)
 	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
-		return New(zones, keys, next, cache)
+		return New(zones, keys, next, ca)
 	})
 
 	// Export the capacity for the metrics. This only happens once, because this is a re-load change only.