MINOR: quic: replace custom buf on Tx by default struct buffer

MINOR: quic: replace custom buf on Tx by default struct buffer

On first prototype version of QUIC, emission was multithreaded. To
support this, a custom thread-safe ring-buffer has been implemented with
qring/cbuf.

Now the thread model has been adjusted : a quic-conn is always used on
the same thread and emission is not multi-threaded. Thus, qring/cbuf
usage can be replace by a standard struct buffer.

The code has been simplified even more as for now buffer is always
drained after a prepare/send invocation. This is the case since a
datagram is always considered as sent even on sendto() error. BUG_ON
statements guard are here to ensure that this model is always valid.
Thus, code to handle data wrapping and consume too small contiguous
space with a 0-length datagram is removed.