Before it was possible that a wallet was able to spend more money than it has. The cause were two pending distributed commits in the system at the same time. Since the wallets only update their ledger when the supervisor sends the commit message, they also agreed to the second request because their ledger was not updated. However, the second request could exceed the rest amount of FUCs in the ledger

#14: delete unused files like Wallet.java, move methods of WalletControl.java interface to AbstractWallet.java and drop WalletControl.java