Refactor roomserver/internal - split perform stuff out (#1380)

- New package `perform` which contains all `Perform` functions
- New package `helpers` which contains helper functions used by both
  perform and query/input functions.
- Perform invite/leave have no idea how to `WriteOutputEvents` and this
  is now returned from `PerformInvite` or `PerformLeave` respectively.

Still to do:
 - RSAPI is fed into the inviter/joiner/leaver - this introduces circular
   logic so will need to be removed.
 - Put query operations in a `query` package.
 - Put input operations (and output) in an `input` package.
 - Factor out helper functions as much as possible, possibly rejigging the
   storage layer in the process.

roomserver/roomserver.go

@@ -47,14 +47,8 @@ func NewInternalAPI(
 		logrus.WithError(err).Panicf("failed to connect to room server db")
 	}
 
-	return &internal.RoomserverInternalAPI{
-		DB:                   roomserverDB,
-		Cfg:                  cfg,
-		Producer:             base.KafkaProducer,
-		OutputRoomEventTopic: string(cfg.Matrix.Kafka.TopicFor(config.TopicOutputRoomEvent)),
-		Cache:                base.Caches,
-		ServerName:           cfg.Matrix.ServerName,
-		FedClient:            fedClient,
-		KeyRing:              keyRing,
-	}
+	return internal.NewRoomserverAPI(
+		cfg, roomserverDB, base.KafkaProducer, string(cfg.Matrix.Kafka.TopicFor(config.TopicOutputRoomEvent)),
+		base.Caches, fedClient, keyRing,
+	)
 }