MINOR: quic: purge txbuf before preparing new packets

MINOR: quic: purge txbuf before preparing new packets

Sending is implemented in two parts on quic-conn module. First, QUIC
packets are prepared in a buffer and then sendto() is called with this
buffer as input.

qc.tx.buf is used as the input buffer. It must always be empty before
starting to prepare new packets in it. Currently, this is guarantee by
the fact that either sendto() is completed, a fatal error is encountered
which prevent future send, or a transient error is encountered and we
rely on retransmission to send the remaining data.

This will change when poller subscribe of socket FD on sendto()
transient error will be implemented. In this case, qc.tx.buf will not be
emptied to resume sending when the transient error is cleared. To allow
the current sending process to work as expected, a new function
qc_purge_txbuf() is implemented. It will try to send remaining data
before preparing new packets for sending. If successful, txbuf will be
emptied and sending can continue. If not, sending will be interrupted.

This should be backported up to 2.7.