[Reviewer: Andy] Omit OIDs if no data to return (rather than returning genError)

Andy,

Please can you review my change to fix https://github.com/Metaswitch/clearwater-infrastructure/issues/59?  (Best to set `?w=1` on the end of the URI.)  There are basically 2 changes here.

*   Don't return `SNMP_ERR_GENERR` if our data is out of date.  Instead, just drop through and return `SNMP_ERR_NOERROR`.
*   Set the last_seen_time to 0 on initialization - it's misleading to say that we have values for the first 15s when we don't really.

I've live tested.

Matt