Move singleflight out of middleware/ (#217)

It is not middleware, so move it up one level. It was also use from
core/ showing that indeed its use is not limited to middlewares.

core/setup/etcd.go

@@ -11,7 +11,7 @@ import (
 	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/etcd"
 	"github.com/miekg/coredns/middleware/proxy"
-	"github.com/miekg/coredns/middleware/singleflight"
+	"github.com/miekg/coredns/singleflight"
 
 	etcdc "github.com/coreos/etcd/client"
 	"golang.org/x/net/context"

middleware/dnssec/dnssec.go

@@ -4,7 +4,7 @@ import (
 	"time"
 
 	"github.com/miekg/coredns/middleware"
-	"github.com/miekg/coredns/middleware/singleflight"
+	"github.com/miekg/coredns/singleflight"
 
 	"github.com/miekg/dns"
 	gcache "github.com/patrickmn/go-cache"

middleware/etcd/etcd.go

@@ -10,7 +10,7 @@ import (
 	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/etcd/msg"
 	"github.com/miekg/coredns/middleware/proxy"
-	"github.com/miekg/coredns/middleware/singleflight"
+	"github.com/miekg/coredns/singleflight"
 
 	etcdc "github.com/coreos/etcd/client"
 	"golang.org/x/net/context"

middleware/etcd/setup_test.go

@@ -11,8 +11,8 @@ import (
 	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/etcd/msg"
 	"github.com/miekg/coredns/middleware/proxy"
-	"github.com/miekg/coredns/middleware/singleflight"
 	"github.com/miekg/coredns/middleware/test"
+	"github.com/miekg/coredns/singleflight"
 
 	etcdc "github.com/coreos/etcd/client"
 	"github.com/miekg/dns"