add option to mocknet to update UTXO set after transactions

[mr-zwets]

currently mocknet doesn't update the utxo set after transactions.

For some testing setups it's preferable that the UTXO set updates after transactions, so we could make this configurable

[kiok46]

It might be a good idea to inspect the outputs of a successfully built transaction, remove the consumed UTXOs, and update the set of available UTXOs with the newly generated outputs.