session: use half-open sessions for vc establishment

session: use half-open sessions for vc establishment

Use half-open sessions to track virtual circuit connection
establishment. These sesssions can only be allocated and freed by the
thread that allocates half-open connections (main). Consequently, they
can only be freed on half-open cleanup notifications from transports.

Goal is to simplify state tracking within the session layer but it's
also a first step towards allowing builtin apps to track and cleanup
outstanding connects.

Type: improvement

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I8a535906d13eb7f8966deb82333839de80f8049f