Make accurate way of consumers balance calculation

[chompomonim]

When in CLI consumer is asking to show balance, this is an ideal time to recalculate "true" balance.

Consumer balance can be calculated this way:
Balance (to spend) = BCBalance - (accountantPromised - BCSettled)

caller, _ := bindings.NewMystTokenCaller()
BCBalance = caller.BalanceOf()

caller, _ := bindings.NewChannelImplementation()
BCSettled = caller.Accountant().Settled

accountantPromised = boltDB.issued_invoices.AgreementTotal

We should also store in boltdb last issued promise. If it's not there, we should ask for own latest promise from the accountant.

Ideally, we could also have "recalculate balance" button in WebUI.