MINOR: quic: Make use of buffer structs to handle STREAM frames

MINOR: quic: Make use of buffer structs to handle STREAM frames

The STREAM data to send coming from the upper layer must be stored until
having being acked by the peer. To do so, we store them in buffer structs,
one by stream (see qcs.tx.buf). Each time a STREAM is built by quic_push_frame(),
its offset must match the offset of the first byte added to the buffer (modulo
the size of the buffer) by the frame. As they are not always acknowledged in
order, they may be stored in eb_trees ordered by their offset to be sure
to sequentially delete the STREAM data from their buffer, in the order they
have been added to it.