Return 503 if the event transfer job isn't keeping up with logged events

[petli]

command.execute/logEvent should check if the event transfer job is keeping up, and if not throw an exception that results in the frontend returning a 503 to the client.

An assumption has to be made by the event rate, and then ensure that say 10 minutes of events fit in the capped collection for temporary event storage in core.

This can then be a fairly simple implementation:

• logEvent need to keep track of the timestamp of the last event logged in the capped collection
• If it checked on the event transfer with the last N minutes (e.g. 2), it will just log the event
• If it has been more than N minutes, it checks the timestamp of the last event in the long-term event collection
  • If that timestamp is > M minutes older (e.g. 5 minutes) than the previously logged event, it rejects the command to return a 503.
    • It remembers this status for K minutes (e.g. 1) and rejects all commands during this period, before checking next if the log transfer is again healthy.
  • Otherwise all is good for another N minutes