MINOR: mux-h1: do not try to receive on backend before sending a request

MINOR: mux-h1: do not try to receive on backend before sending a request

There's no point trying to perform an recv() on a back connection if we
have a stream before having sent a request, as it's expected to fail.
It's likely that this may avoid some spurious subscribe() calls in some
keep-alive cases (the close case was already addressed at the connection
level by "MINOR: connection: avoid a useless recvfrom() on outgoing
connections").