session: application namespace may reference a deleted vrf table

session: application namespace may reference a deleted vrf table

lock the vrf table when adding an application namespace and
unlock the vrf table when deleting an application namespace.

Free the session table when no more application namespace
uses it anymore to avoid memory leaks.

Type: fix

Change-Id: I10422c9a3b549bd4403962c925e29dd61a058eb0
Signed-off-by: Steven Luong <sluong@cisco.com>