Changes the accounts_balances RPC to return per account results

[thsfs]

The previous implementation returned only an error result in case any of the accounts were invalid. This implementation now handles each account separately returning per account results.

    "balances": {
        "nano_3wfddg7a1paogrcwi3yhwnaerboukbr7rs3z3ino5toyq3yyhimo6f6egij6": {
            "balance": "442000000000000000000000000000",
            "pending": "0",
            "receivable": "0"
        },
        "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz": {
            "error": "Bad account number"
        },
        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": {
            "error": "Account not found"
        }
    }

Reference issue: #3752

[zhyatt]

Since we are doing Account not found error in some of the other responses when the account hasn't been opened, such as in #3789 , should we also do that here instead of 0 values? @clemahieu @thsfs @dsiganos

        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": {
            "error": "Account not found"
        },

[thsfs]

Since we are doing Account not found error in some of the other responses when the account hasn't been opened, such as in #3789 , should we also do that here instead of 0 values? @clemahieu @thsfs @dsiganos

        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": {
            "error": "Account not found"
        },

It makes sense to me to have the same type of error response when the account is not found.

[dsiganos]

This RPC should also be taking include_only_confirmed as option.
Like account_balance does. But that could be a different PR.