Do not try to re-send saved events if the observer is no longer registered

[obycode]

Problem

• Event observer X is registered on a stacks-node
• Event observer X goes down, so the stacks-node is stuck, attempting to successfully send an event to X
• X is no longer needed, so it is removed from the stacks-node's configuration
• The event attempted to send to X was recorded in the event_observers.sqlite, so on the next event, the node attempts again to send the event to X

Solution

When processing pending payloads from the database, we should first check if the event observer is still registered for this node. If it is not, just drop the pending payload without attempting to send it.

[obycode]

Alternatively, this could be implemented as a cleaning of the pending payloads database on startup (or when the event observers change).