Initial Store & Forward Implementation (#2917)

This adds store & forward relays into dendrite for p2p.
A few things have changed:
- new relay api serves new http endpoints for s&f federation
- updated outbound federation queueing which will attempt to forward
using s&f if appropriate
- database entries to track s&f relays for other nodes

relayapi/api/api.go

@@ -0,0 +1,56 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package api
+
+import (
+	"context"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// RelayInternalAPI is used to query information from the relay server.
+type RelayInternalAPI interface {
+	RelayServerAPI
+
+	// Retrieve from external relay server all transactions stored for us and process them.
+	PerformRelayServerSync(
+		ctx context.Context,
+		userID gomatrixserverlib.UserID,
+		relayServer gomatrixserverlib.ServerName,
+	) error
+}
+
+// RelayServerAPI exposes the store & query transaction functionality of a relay server.
+type RelayServerAPI interface {
+	// Store transactions for forwarding to the destination at a later time.
+	PerformStoreTransaction(
+		ctx context.Context,
+		transaction gomatrixserverlib.Transaction,
+		userID gomatrixserverlib.UserID,
+	) error
+
+	// Obtain the oldest stored transaction for the specified userID.
+	QueryTransactions(
+		ctx context.Context,
+		userID gomatrixserverlib.UserID,
+		previousEntry gomatrixserverlib.RelayEntry,
+	) (QueryRelayTransactionsResponse, error)
+}
+
+type QueryRelayTransactionsResponse struct {
+	Transaction   gomatrixserverlib.Transaction `json:"transaction"`
+	EntryID       int64                         `json:"entry_id"`
+	EntriesQueued bool                          `json:"entries_queued"`
+}