Fix the import statements in etcd.go

Can't use this workaround
(etcd-io/etcd#11563 (comment))
if valkeyrie mixes go.etcd.io/etcd with github.com/coreos/etcd for some reason.

store/etcd/v3/etcd.go

@@ -10,7 +10,7 @@ import (
 	"github.com/abronan/valkeyrie"
 	"github.com/abronan/valkeyrie/store"
 	etcd "github.com/coreos/etcd/clientv3"
-	"go.etcd.io/etcd/clientv3/concurrency"
+	"github.com/coreos/etcd/clientv3/concurrency"
 )
 
 const (