In multiplexing, we have a single connector type mapper, meaning that type handlers are used concurrently. We should check that all handlers are compatible with this; it should generally be the case (they were written with this in mind), but we should be sure.

Specifically, NetTopologySuite uses external PostgisReader/PostgisWriter types whose type safety isn't clear. They also suffer from some other issues (e.g. #2598), so we might be better off implementing the I/O logic ourselves etc.