bugfix: E2EE device keys could sometimes not be sent to remote servers (#2466)

* Fix flakey sytest 'Local device key changes get to remote servers'

* Debug logs

* Remove internal/test and use /test only

Remove a lot of ancient code too.

* Use FederationRoomserverAPI in more places

* Use more interfaces in federationapi; begin adding regression test

* Linting

* Add regression test

* Unbreak tests

* ALL THE LOGS

* Fix a race condition which could cause events to not be sent to servers

If a new room event which rewrites state arrives, we remove all joined hosts
then re-calculate them. This wasn't done in a transaction so for a brief period
we would have no joined hosts. During this interim, key change events which arrive
would not be sent to destination servers. This would sporadically fail on sytest.

* Unbreak new tests

* Linting

test/testrig/jetstream.go

@@ -0,0 +1,35 @@
+package testrig
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/setup/base"
+	"github.com/matrix-org/dendrite/setup/jetstream"
+	"github.com/nats-io/nats.go"
+)
+
+func MustPublishMsgs(t *testing.T, jsctx nats.JetStreamContext, msgs ...*nats.Msg) {
+	t.Helper()
+	for _, msg := range msgs {
+		if _, err := jsctx.PublishMsg(msg); err != nil {
+			t.Fatalf("MustPublishMsgs: failed to publish message: %s", err)
+		}
+	}
+}
+
+func NewOutputEventMsg(t *testing.T, base *base.BaseDendrite, roomID string, update api.OutputEvent) *nats.Msg {
+	t.Helper()
+	msg := &nats.Msg{
+		Subject: base.Cfg.Global.JetStream.Prefixed(jetstream.OutputRoomEvent),
+		Header:  nats.Header{},
+	}
+	msg.Header.Set(jetstream.RoomID, roomID)
+	var err error
+	msg.Data, err = json.Marshal(update)
+	if err != nil {
+		t.Fatalf("failed to marshal update: %s", err)
+	}
+	return msg
+}