Create weak `WriteWrap` and `ShutdownWrap` objects, and when
referencing them in C++ is necessary, use `BaseObjectPtr<>`
instead of plain pointers to keep these objects from being
garbage-collected.

This solves issues that arise when the underlying `StreamBase`
instance is weak, but the `WriteWrap` or `ShutdownWrap` instances
are not; in that case, they would otherwise potentially stick
around in memory after the stream that they originally belong
to is long gone.

It probably makes sense to use `BaseObjectptr<>` more extensively
in `StreamBase` in the long run as well.