This function always appends to an existing string, so by passing it a
String directly we can avoid superfluous allocations.
Also tokio-xmpp was doing a bunch of &str to String for no reason around
there, let’s remove that too.
Instead of having a second method to fetch channel-binding from the
TlsStream, do it directly in the connect() method, since after that we
don’t have enough information to fetch it any longer when using ktls.
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.
