New NoticeStream class to reify streams of notices

New NoticeStream class to reify streams of notices

We've been muddling through with 6- or 8-argument functions for managing streams. I'd
like to start thinking of streams as their own thing, and give them some more value.

So, the new NoticeStream class takes over the Notice::stream() function and Notice::getStreamByIds().

There's probably some fine-tuning to do on the object interface.