MEDIUM: quic: implement refcount for quic_conn

MEDIUM: quic: implement refcount for quic_conn

Implement a refcount on quic_conn instance. By default, the refcount is
0. Two functions are implemented to manipulate it.
* qc_conn_take() which increments the refcount
* qc_conn_drop() which decrements it. If the refcount is 0 *BEFORE*
  the substraction, the instance is freed.

The refcount is incremented on retrieve_qc_conn_from_cid() or when
allocating a new quic_conn in qc_lstnr_pkt_rcv(). It is substracted most
notably by the xprt.close operation and at the end of
qc_lstnr_pkt_rcv(). The increments/decrements should be conducted under
the CID lock to guarantee thread-safety.