Add check to Custodian.inspectWalletTx to avoid re-inserting existing asset into local db

[ffranr]

Custodian.inspectWalletTx should check to see if an asset (coin) and its corresponding proof already exists in the local db before attempting to import again. This method is actually call before the Custodian event loop here. At that point, on tapd restart, all wallet transactions are re-inspected.

We should also refactor the events cache so that it's a thread safe events processing queue. This is because events added during the custodian lifetime are handled synchronously and are not cleaned up from the events cache. And the current form of events is difficult to refactor.

[guggero]

Fixed by #836 by skipping already completed events.