Refactor session storage

Session storage had a few bugs, and a number of old practices.  This
moves the session storage closer to the best practices.  It enforces
the use of a factory function for generating new sessions, as well as
using get_ptr when reading the sessions out.

Change-Id: Ia252076f21e47b99f8057190349355838fdd787d
Signed-off-by: Ed Tanous <ed.tanous@intel.com>