BUG/MINOR: mux-h2: refrain from muxing during the preface

BUG/MINOR: mux-h2: refrain from muxing during the preface

The condition to refrain from processing the mux was insufficient as it
would only handle the outgoing connections. In essence it is not that much
of a problem since we don't have streams yet on an incoming connetion. But
it prevents waiting for the end of the preface before sending an early
WINDOW_UPDATE message, thus causing the connections to fail in this case.

This must be backported to 1.8 with a few minor adaptations.