Connection closure in handler

[arnetheduck]

In protocol handlers, there's a zoo of options with regards to closing the stream after handling the protocol request:

• example says handler must conn.close
• identify uses closeWithEOF
• ping doesn't close at all

Multistream closes the connection after the handling is done, so it seems like closing is redundant / not needed but if that's the case, at least the example should be updated, but probably also all existing handlers, so they behave the same - the semantic difference to capture here is whether closing should be a fire-and-forget option or whether we want to wait for a graceful two-sided shutdown - if we close in multistream, the handler has no way of knowing if the other side closed gracefully.

A complication is of course is what to do when an exception is raised and partial data has been written - typically, it should be expected that handlers handle their exceptions also, but it so turns out that multistream swallows all exceptions silently potentially leading to undetected problems.