middleware/cache: split cache in positive and negative and use lru (#298)

Make the cache memory bounded, by using a LRU cache. Also split the
cache in a positive and negative one - each with its own controls.

Extend the cache stanza to allow for this:

    cache {
       positive limit [ttl]
       negative limit [ttl]
    }

is now possible. This also add a cache_test.go in the toplevel test/
directory that exercises the caching path.

Fixes #260

test/middleware_dnssec_test.go

@@ -31,7 +31,7 @@ func TestLookupBalanceRewriteCacheDnssec(t *testing.T) {
 `
 	ex, err := CoreDNSServer(corefile)
 	if err != nil {
-		t.Fatalf("could not get CoreDNS serving instance: %s", err)
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
 	}
 
 	udp, _ := CoreDNSServerPorts(ex, 0)