CLEANUP: quic: improve naming for rxbuf/datagrams handling

CLEANUP: quic: improve naming for rxbuf/datagrams handling

QUIC datagrams are read from a random thread. They are then redispatch
to the connection thread according to the first packet DCID. These
operations are implemented through a special buffer designed to avoid
locking.

Refactor this code with the following changes :
* <rxbuf> type is renamed <quic_receiver_buf>. Its list element is also
  renamed to highligh its attach point to a receiver.
* <quic_dgram> and <quic_receiver_buf> definition are moved to
  quic_sock-t.h. This helps to reduce the size of quic_conn-t.h.
* <quic_dgram> list elements are renamed to highlight their attach point
  into a <quic_receiver_buf> and a <quic_dghdlr>.

This should be backported up to 2.6.