vcl: add QUIC support

vcl: add QUIC support

Type: feature

* Adds the concept of a "connectable listener" : a session that
 can be both connected and accepted on.
* vppcom_session_is_connectable_listener (fd) that tells if the fd
is a connectable listener
* vppcom_session_listener (fd) that gives you the listener's fd
that accepted the session (if any)
* vppcom_session_n_accepted (fd) that gives the number
of sessions a listener accepted.

Change-Id: Id89d67d8339fb15a7cf7e00a9c5448175eca04fc
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>