MINOR: mux-quic: define Tx connection buffer for QMux

MINOR: mux-quic: define Tx connection buffer for QMux

Similarly to reception, a new buffer is defined in QCC connection to
handle emission for QMux protocol. This replaces the trash buffer usage
in qcc_qstrm_send_frames().

This buffer is necessary to handle partial emission. On retry, the
buffer must be completely emitted before starting to send new frames.