PR #986 introduced two distinct balances at different levels of
the wallet tree; it serialized these balances for CLI and Qt display
but changed the RPC-API json output of the /display endpoint in a
suboptimal way. This commit fixes the json structure.
After this commit, the four levels (wallet, account, branch, entry) now
all still have balances reported under the same key as before #986, that
is, total_balance, account_balance, balance and amount, but the extra
information introduced by that PR, namely the 'available_balance' is
added as an extra key in the json dict.
Tests are added to check this structure.
Adam Gibson
08581f86a5