Filter /members, return members at given point (#2827)

Makes the tests
```
Can get rooms/{roomId}/members at a given point
Can filter rooms/{roomId}/members
```
pass, by moving `/members` and `/joined_members` to the SyncAPI.

syncapi/storage/interface.go

@@ -178,6 +178,11 @@ type Database interface {
 	ReIndex(ctx context.Context, limit, afterID int64) (map[int64]gomatrixserverlib.HeaderedEvent, error)
 	UpdateRelations(ctx context.Context, event *gomatrixserverlib.HeaderedEvent) error
 	RedactRelations(ctx context.Context, roomID, redactedEventID string) error
+	SelectMemberships(
+		ctx context.Context,
+		roomID string, pos types.TopologyToken,
+		membership, notMembership *string,
+	) (eventIDs []string, err error)
 }
 
 type Presence interface {