Add additional check when receiving Deregistration Notification from
UDM. UE should already be in registered state before accepting the
request and deregistering the UE.
Also add additional check that PCF association policy exists before
sending a delete request to PCF.
UDM may send a Deregistration Notification to AMF, to deregister
specific UE from the network - Network-Initiated Deregistration.
Deregistration procedure includes sending Deregistration Request to UE,
starting a timer T3522, releasing PDU sessions from SMF, releasing PCF
policies from PCF, and waiting for Deregistration Accept from UE.
Not yet implemented is:
- to prevent deregistration of UE in case it has any emergency sessions,
- page UE when UE is in IDLE mode.
All process will be forcely exited if it failed to encode the S1AP/NGAP/GTP/PFCP message. It is to make sure there was no problem with the encoding of open5gs.