[indexer-balances] Sanity check data against RPC when performing DB only calculations

[morgsmccauley]

Rather than using RPC to calculate the balance change, #45 uses the data which already exists in the DB. The problem with this approach is that errors will continuously propagate through following calculations, as new calculations use previous values. This is not a problem with the RPC approach as new calculations rely on data coming directly from the network, which will always be correct. Errors will be isolated to a single entry in the DB.

To mitigate this problem, we should introduce some form of external process which performs random sanity checks of the DB against RPC. This allows us to be notified of, and respond to, errors early.