If both Delete-Session-Request/Response and
UEContextReleaseCommand/UEContextReleaseComplete are failed at the same time,
UE cannot attach to the EPC infinitely.
So, I've add the protection code
if timer expires when MME does not receive Delete-Session-Response.
So far, no operation was performed when Error Indication was received
from eNodeB. For that reason, I solved #568 issues by controlling
the MME to prevent this from happening.
Now, when GTP-U Error Indication is received, MME and SGW are implemented
to do what they have to do. I hope that the network can be restored
by responding appropriately even if Error Indication occurs.
