queue and verify submitted events

[BrightonBTC]

Submitted events should be added to an events queue - probably a db table so that it persists if the app is shut down - that waits for an OK message to be received from the preferred relay.

This means that if the submission fails we can try again after x minutes

When the OK message is received we can either delete the entry or possibly we could extend the use of this table to add 'seen on' functionality.