mme: Trigger last parts of procedure TC_ue_cell_reselect_eutran_to_geran

Some later parts of the 4G->2G cell reselection where missing, mostly
related to tear down after MS has successfuly moved to the new SGSN. mainly:
- S6a Diameter Cancel Location Req + Answer initiated by HSS.
- Handle UeReleaseContextCommand and answer with
  UeReleaseContextComplete, emulating the ENB.
- handle S11 GTPv2C Delete Session Request and answer with Delete
  Session Response, emulating the SGW.

Change-Id: Ic0263a3aec922194aad22b031b2c82a99954354d
1 file changed