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.

etcd/chooser/chooser.go

@@ -22,7 +22,7 @@ import (
 
 	"github.com/purpleidea/mgmt/etcd/interfaces"
 
-	etcdtypes "github.com/coreos/etcd/pkg/types"
+	etcdtypes "go.etcd.io/etcd/pkg/types"
 )
 
 // Data represents the input data that is passed to the chooser.

etcd/chooser/dynamicsize.go

@@ -25,8 +25,8 @@ import (
 
 	"github.com/purpleidea/mgmt/etcd/interfaces"
 
-	etcd "github.com/coreos/etcd/clientv3"
-	etcdtypes "github.com/coreos/etcd/pkg/types"
+	etcd "go.etcd.io/etcd/clientv3"
+	etcdtypes "go.etcd.io/etcd/pkg/types"
 )
 
 // XXX: Test causing cluster shutdowns with: