Add a way to force federationsender to retry sending transactions (#1077)

* Add a way to force federationsender to retry sending transactions

And use it in P2P mode when we pick up new nodes.

* Linting

* Use atomic bool to stop us blocking on the channel

federationsender/api/api.go

@@ -42,6 +42,12 @@ type FederationSenderInternalAPI interface {
 		request *PerformLeaveRequest,
 		response *PerformLeaveResponse,
 	) error
+	// Notifies the federation sender that these servers may be online and to retry sending messages.
+	PerformServersAlive(
+		ctx context.Context,
+		request *PerformServersAliveRequest,
+		response *PerformServersAliveResponse,
+	) error
 }
 
 // NewFederationSenderInternalAPIHTTP creates a FederationSenderInternalAPI implemented by talking to a HTTP POST API.