Refactor StoreEvent and create a new RoomDatabase interface (#2985)

This PR changes a few things:
- It pulls out the creation of several NIDs from the `StoreEvent`
function to make the functions more reusable
- Uses more caching when using those NIDs to avoid DB round trips

internal/caching/cache_roomservernids.go

@@ -9,19 +9,28 @@ type RoomServerCaches interface {
 	RoomVersionCache
 	RoomServerEventsCache
 	EventStateKeyCache
+	EventTypeCache
 }
 
 // RoomServerNIDsCache contains the subset of functions needed for
 // a roomserver NID cache.
 type RoomServerNIDsCache interface {
 	GetRoomServerRoomID(roomNID types.RoomNID) (string, bool)
+	// StoreRoomServerRoomID stores roomNID -> roomID and roomID -> roomNID
 	StoreRoomServerRoomID(roomNID types.RoomNID, roomID string)
+	GetRoomServerRoomNID(roomID string) (types.RoomNID, bool)
 }
 
 func (c Caches) GetRoomServerRoomID(roomNID types.RoomNID) (string, bool) {
 	return c.RoomServerRoomIDs.Get(roomNID)
 }
 
+// StoreRoomServerRoomID stores roomNID -> roomID and roomID -> roomNID
 func (c Caches) StoreRoomServerRoomID(roomNID types.RoomNID, roomID string) {
+	c.RoomServerRoomNIDs.Set(roomID, roomNID)
 	c.RoomServerRoomIDs.Set(roomNID, roomID)
 }
+
+func (c Caches) GetRoomServerRoomNID(roomID string) (types.RoomNID, bool) {
+	return c.RoomServerRoomNIDs.Get(roomID)
+}