[UPDATE] Getbalance wrapper

We need to wrap our getbalance call due to issues in the coind and
payout systems in the RPC. Apparently it's having issues with multiple
accounts setup in a single wallet. Sendmany can not properly use other
accounts added to the wallet as the payout account.

Hence we wrap our getbalance call in the RPC and only return the default
account balance at all times. Since this is used for all payouts and for
payments from blocks, there should be no issue. Those pools running
multiple accounts in their wallet will have to move their coins into the
default wallet!