OPTIM: quic: do not call qc_send() if nothing to emit

OPTIM: quic: do not call qc_send() if nothing to emit

qc_send() was systematically called by quic_conn IO handlers with all
instantiated quic_enc_level. Change this to only register quic_enc_level
for send if needed. Do not call at all qc_send() if no qel registered.

A new function qel_need_sending() is defined to detect if sending is
required. First, it checks if quic_enc_level has prepared frames or
probing is set. It can also returns true if ACK required either on
quic_enc_level itself or because of quic_conn ack timer fired. Finally,
a CONNECTION_CLOSE emission for quic_conn is also a valid case.

This should reduce the number of invocations of qc_send(). This could
improve slightly performance, as well as simplify traces debugging.