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/types/types.go

@@ -234,6 +234,9 @@ func (t *TopologyToken) StreamToken() StreamingToken {
 }
 
 func (t TopologyToken) String() string {
+	if t.Depth <= 0 && t.PDUPosition <= 0 {
+		return ""
+	}
 	return fmt.Sprintf("t%d_%d", t.Depth, t.PDUPosition)
 }