Changed stake total to use debit instead of credit.

Changed CWallet::GetStake to total Debit instead
of Credit for immature POS blocks generated by
the user. Also changed the name to GetStakedCoin
to make it easier to track down who calls this
function.

Since immature stake interest transactions are
included in the immature total, they don't
belong in the stake total. This will prevent
the user from thinking the stake interest
disappeared from their wallet after a POS
transaction matures.