Incorrect way of prunning the retransmission queue.

[QUICTester]

Hi, During testing commit https://github.com/p-quic/pquic/commit/841c82289b23d32c28723a0175654ecc6baeae08, we discovered that the PQUIC server uses Picoquic code that does not correctly prune the retransmission queue in picoquic/sender.c:picoquic_implicit_handshake_ack().

The pruning direction on the retransmission queue is opposite to how the packets are added in picoquic/sender.c:picoquic_queue_for_retransmit().

Therefore the picoquic_packet_t* p_next = p->next_packet; in picoquic/sender.c:picoquic_implicit_handshake_ack() should be picoquic_packet_t* p_next = p->previous_packet;

[mpiraux]

Could you test #39 as a potential fix ?

[QUICTester]

Yes, it does not have any problem anymore when we test.