fix: issue 911 - OAI gNB hangs when AMF connection goes down

Once the AMF is down, based on the AMF UE NGAP ID and RAN UE NGAP ID,
the UE associated with the AMF is found

All the contexts related to the UE are released and the timer is started
in the NGAP to look up for the SCTP association. The AMF-gNB reconnection
is made if an SCTP connection is found, also the registration of multiple
UE is triggered

These actions are based on the reference from 4.2.6 AN release Specs
TS 123 502 V16.7.0 (2021-01)

When the NG-AP signalling connection is lost due to (R)AN or AMF failure,
the AN release is performed locally by the AMF or the (R)AN as described
in the procedure flow below without using or relying on any of the signalling
shown between (R)AN and AMF. The AN release causes all UP connections of the UE
to be deactivated.

closes #911