This allows to use any serialisable type. The advantage is that moves
and clones are avoided (which would otherwise be needed to construct
e.g. a XmppStreamElement from a Stanza or Message).
This allows to detect and handle dying streams without getting stuck
forever.
Timeouts are always wrong, though, so we put the burden of choosing the
right values (mostly) on the creator of a stream.
This makes the stream resets a lot safer, by preventing the forbidden
send-read-reset combination of events: the reset function on the
responder side now takes the element to send right before the reset,
enforcing a send-reset pattern.
