Avoid unnecessary calculations in propose_batch

[elderhammer]

Determine in advance whether the proposal has expired.
If it expires, return directly and skip the subsequent process.

[vicsn]

Interesting find. Perhaps the following should also be moved up for consistency:

// Ditto if the batch had already been proposed and not expired.
ensure!(round > 0, "Round 0 cannot have transaction batches");

[elderhammer]

Interesting find. Perhaps the following should also be moved up for consistency:

// Ditto if the batch had already been proposed and not expired.
ensure!(round > 0, "Round 0 cannot have transaction batches");

Done.

[raychu86]

LGTM