[Serusers] Missed Call Return

[Andreas Granig]

Iqbal wrote:
> nice, how do you pick the last number only, or would it be better when 
> you stored a missed call, that you deleted other entries in the table.

A valid objection... in this case it's just luck, because avps are 
pushed in reverse order. And SER logs an error if the same caller calls 
the callee twice because user/domain/attribute/value is the primary key.

So you're right, it would be nicer to call avp_db_delete before 
avp_db_store in failure_route.

Andy