This commit introduces the internal accounting logic for accrueing
multiplier points, that will later be used to determine how many
experience points an account is eligible to.
The majority of the work here was done by @3esmit.
This primarily adds a rule that ensures that, when an account's
`balance` changes, `_processAccount()` must have been called as well.
There's very few exceptions where an account's `balance` can change
without the need of `_processAccount()` but those functions have been
deliberately excluded from the rule.
