MINOR: quic: Move some counters from [rt]x quic_conn anonymous struct

MINOR: quic: Move some counters from [rt]x quic_conn anonymous struct

Move rx.bytes, tx.bytes and tx.prep_bytes quic_conn struct member to
bytes anonymous struct (bytes.rx, bytes.tx and bytes.prep member respectively).
They are moved before being defined into a bytes anonoymous struct common to
a future struct to be defined.

Consequently adapt the code.