twap order may expire earlier if span is set

[banteg]

if span is set to a non-zero value, the last part of the conditional order would expire after span and not after t.
optimize the calculation of expiration timestamp for such cases.

cow-sdk/src/composable/orderTypes/Twap.ts

Line 310 in 228ff3e

const expirationTimestamp = startTimestamp + numberOfParts.mul(timeBetweenParts).toNumber()