Ensure Keyshare and Aggregators Destroy themselves

[ryardley]

Keyshares and Aggregators should destroy themselves once completed to avoid memory leaks
We can use the event hooks to look out for events to deregister them selves!

[ryardley]

There is a broader question here about in memory-db-lifecycle. We need to be able to resuscitate actor state from the db in the event of a failure - every actor state change should be lazily saved. - wrote more about this here