fix: Make batch shares rounding favour the system

[bingen]

It uses ceil division in TroveManager._updateBatchShares, so that the rounding error goes against the user instead of the rest of the batch.

• Pros
  • It seems safer, as it would prevent the flagged issues of borrowing for free
  • It’s more consistent with the other direction
• Cons
  • It can still be gamed: other batch users may frontrun a borrowing operation, so that the user increasing debt owes more than minted due to rounding error.
  • The impact may be higher: now a user would swallow all the rounding error if frontrun (otherwise, the user can choose the right amount to prevent it), while before the error would be shared among the rest of the batch (most cases that would be higher, unless a user has >= 50% of the batch)
  • Introducing changes so late always poses some risk, and it seems the rounding issues have been quite mitigated by Prevent shares to debt ratio in batches to be too low #455