Denormalise net user debts to optimise algorithm to minimise transactions

[IsaacCheng9]

Specification

• As mentioned in PR #58, the algorithm for minimising transactions is currently O(k * n log n), where k is the number of debts, and n is the number of users.
• We can optimise the algorithm to O(n log n) by denormalising the user debts.
  • The algorithm currently spends O(k) time going through all debts and calculating the net debt for each user.
  • If we store this in a separate collection and update it every time a debt pair is updated, we'll save repeated work.