Skip waiting for signed boarding ins/forfeit txs if none is required

[altafan]

This includes server-side optimization for the execution of the round:

• Do not wait for the execution timeout to expire when a round doesn't contain boarding inputs or forfeit txs to be signed: this is a very edge case in which a round includes only users that are redeeming notes. In this case, users just need to sign the VTXO tree to complete the round execution. Before this, in such case, the server would have waited for the timeout to expire because clients wouldn't submit any signed boarding input or forfeit tx.
• Add safe checks to SignRoundTx to prevent overriding a round tx if the user didn't submit any signed boarding input. Before this, a malicious client could have submitted an unsigned round tx to make the server loose all previous collected signed boarding inputs.

Closes #492.

Please @louisinger @sekulicd review.