Gb: implement MM state ready, idle, standby including T3314
MM state reflects the knowledge of the MS' location in the network.
Implement the state and add the hooks so the states are tracked.
READY: exact cell is known
STANDBY: RA known
IDLE: unknown location
On receivement of a DL PDU for the MS, the SGSN has to:
READY: send DL PDU to PCU (already done)
STANDBY: page MS in RA (seperate ticket)
IDLE: deactivate pdp context? (check spec)
T3314 defines the time while the MS is in READY state. When T3314 expires the MM state is set to STANDBY.