refactor: remove duplicate amounts, bug fixes

[ZeroWave022]

• Fixes wrong usdValue in Amount
• Removes duplicate amountsTransaction.usdValue and Wallet.usdValue. Use Transaction.amount.usdValue and Wallet.balance.usdValue instead
• Improved Amount.toString
• Rewrites parts of RequestHandler

[paulofreitas]

While this has already been merged, I should make some notes:

• Both Transaction.usd_value and Wallet.usd_value are part of the API schema - the fact that you also have Amount.usdValue is merely an implementation detail to bring utility to Amount instances as they are one of the most important models on the API
• Transaction.usd_value is the USD value of the transaction at the time it occurred - it will never be the same as Transaction.amount.usdValue, the current USD value of the same amount. Both values ​​are useful - you need the first to know if the transaction is in profit or loss
• Wallet.usd_value should be the same as Wallet.amount.usdValue, but as mentioned earlier, it's part of the schema - you usually bring API data as is, you don't mess with them 👍