[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*   Don't return `SNMP_ERR_GENERR` if our data is out of date.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
Can we at least log out the mode we were asked for?