add transaction discarded event

[Neotamandua]

Summary

Needed by #2600 and also needed to properly handle any case during a transaction's lifecycle. Right now, you can only listen for a Removed(Hash) event and then assume that it will be discarded at some point because there will never be an Executed(&'t SpentTransaction) event. This is not something you should rely on.

Possible Solution (optional)

Add a discarded event type to the node data TransactionEvent and sent this event out for cases where a transaction is discarded. Document the discarded event to include what "discarded" means and what cases trigger a transaction to be discarded.

[Neotamandua]

We don't need this event if it is clear that a transaction will only be discarded if it is invalid for the protocol. This means that we only need to make sure that a transaction is never discarded if it is created by our wallets or by our SDKs, since these implementations comply with the protocol specifications.

Invalid means that the transaction payload is incorrect (e.g., missing or malformed data, or a minimum gas limit below the set minimum).