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

roomserver/types/types.go

@@ -38,6 +38,11 @@ type EventNID int64
 // RoomNID is a numeric ID for a room.
 type RoomNID int64
 
+type EventMetadata struct {
+	EventNID EventNID
+	RoomNID  RoomNID
+}
+
 // StateSnapshotNID is a numeric ID for the state at an event.
 type StateSnapshotNID int64