MINOR: quic: extract datagram parsing code

MINOR: quic: extract datagram parsing code

Extract individual datagram parsing code outside of datagrams list loop
in quic_lstnr_dghdlr(). This is moved in a new function named
quic_dgram_parse().

To complete this change, quic_lstnr_dghdlr() has been moved into
quic_sock source file : it belongs to QUIC socket lower layer and is
directly called by quic_sock_fd_iocb().

This commit will ease implementation of quic-conn owned socket.
New function quic_dgram_parse() will be easily usable after a receive
operation done on quic-conn IO-cb.

This should be backported up to 2.7.