Mark DF as failed if timestamps don't differ

[ngoerlitz]

Currently, the cache only marks the datafeed as "failed", if there has been a rapid decrease in pilots, or the request to VATSIM's API failed outright.
Add a check to verify that the last timestamp of the datafeed is different to the current one. To allow for some discrepancy, we should trigger this at around a minute or two. I.e. if the current time is >= 120secs off the last update_timestamp, then mark the DF as "failed".