Implement handler extension to TTL-expire events

Some events will only fire once and they're gone, there will be no continuous stream. We still want to alert on them but not require a human to resolve the event. Some examples are events originating from log files or events coming from hardware traps.

This extension allows an event to opt-into a self-expiration time by including the field `ttl` with a delay in seconds. The check name and delay get stored in the stash.

The extension also sets up a periodic review of the stash for expired checks, which are then resolved and their TTL entries are removed.