Enhance fee transaction sanity checks

[jholdstock]

Fee transactions received from clients are stored in the DB and broadcast after the ticket purchase is confirmed. The transactions are sanity checked upon receipt - e.g. is the transaction structure valid, and does it pay to the correct fee address?

The current checks are somewhat limited because they are context free; they do not consider the current state of the blockchain and the UTXO set. Adding some contextual checks can reduce the chance of vspd accepting a tx which will fail to broadcast - e.g. have any of the inputs to the transaction already been spent?

[jholdstock]

I think we can return an error if an input is already spent, but i dont think we can return an error if it is non-existent.

E.g. One of the inputs for the fee could be change from the ticket purchase, so vspd might not be able to see it immediately but it would become valid after the ticket purchase is confirmed.

What do you think @jrick?

[jrick]

I think we can return an error if an input is already spent, but i dont think we can return an error if it is non-existent.

The two are the same thing as far as dcrd is concerned. A transaction with all outputs spent will return the same error from gettxout as querying a transaction that doesn't exist.

I don't think ticket change is something we need to deal with. dcrwallet never creates SSTXCHANGE outputs, and pays the VSP fee from earlier regular transactions.