Rendevous transports have a "x_vc_data", and can use it to
determine when a shared transport can't be shared (because it
was marked destroyed).  The x_vc_data wasn't being freed
when the associated duplex record had its refcount go to 0.
This change tracks x_vc_data so it can be freed at the proper time.

Signed-off-by: Marcus Watts <mdw@cohortfs.com>