BUG/MINOR: quic: ensure Tx buf is always purged

BUG/MINOR: quic: ensure Tx buf is always purged

quic_conn API for sending was recently refactored. The main objective
was to regroup the different functions present for both handshake and
application emission.

After this refactoring, an optimization was introduced to avoid calling
qc_send() if there was nothing new to emit. However, this prevent the Tx
buffer to be purged if previous sending was interrupted, until new
frames are finally available.

To fix this, simply remove the optimization. qc_send() is thus now
always called in quic_conn IO handlers.

The impact of this bug should be minimal as it happens only on sending
temporary error. However in this case, this could cause extra latency or
even a complete sending freeze in the worst scenario.

This must be backported up to 3.0.