MINOR: quic-be: ->connect() protocol callback adaptations

MINOR: quic-be: ->connect() protocol callback adaptations

Modify quic_connect_server() which is the ->connect() callback for QUIC protocol:
    - add a BUG_ON() run when entering this funtion: the <fd> socket must equal -1
    - conn->handle is a union. conn->handle.qc is use for QUIC connection,
      conn->handle.fd must not be used to store the fd.
    - code alignment fix for setsockopt(fd, SOL_SOCKET, (SO_SNDBUF|SO_RCVBUF))
	  statements
    - remove the section of code which was duplicated from ->connect() TCP callback
    - fd_insert() the new socket file decriptor created to connect to the QUIC
      server with quic_conn_sock_fd_iocb() as callback for read event.