etcd: Bump to new 3.4.x version

This moves to the newest etcd release, and also updates the imports to
the new go.etcd.io path. I think this is a bit of a pain, but might as
well get it done.

lib/main.go

@@ -45,7 +45,7 @@ import (
 	"github.com/purpleidea/mgmt/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 
-	etcdtypes "github.com/coreos/etcd/pkg/types"
+	etcdtypes "go.etcd.io/etcd/pkg/types"
 )
 
 const (