Room server changes for room versions (#930)

* Rearrange state package a bit, add some code to look up the right state resolution algorithm

* Remove shared

* Add GetRoomVersionForRoomNID

* Try to use room version to get correct state resolution algorithm

* Fix room joins over federation

* nolint resolveConflictsV2 because all attempts to break it up so far just result in it being awfully less obvious how it works

* Rename Prepare to NewStateResolution

* Update comments

* Re-add missing tests

roomserver/storage/postgres/storage.go

@@ -747,6 +747,14 @@ func (d *Database) GetRoomVersionForRoom(
 	)
 }
 
+func (d *Database) GetRoomVersionForRoomNID(
+	ctx context.Context, roomNID types.RoomNID,
+) (gomatrixserverlib.RoomVersion, error) {
+	return d.statements.selectRoomVersionForRoomNID(
+		ctx, nil, roomNID,
+	)
+}
+
 type transaction struct {
 	ctx context.Context
 	txn *sql.Tx