Changes the accounts_frontiers 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. It returns an empty string when the account's frontier is zero, and returns an error message for when the provided account is invalid.

Example:

    "frontiers": {
        "nano_3wfddg7a1paogrcwi3yhwnaerboukbr7rs3z3ino5toyq3yyhimo6f6egij6": "75BD65296241EB871918EBE3E99E9A191970A2724B3214B27F8AB205FF4FC30A",
        "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz": "error: Bad account number",
        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": "error: Account not found"
    }

Reference issue: #3752

[dsiganos]

The "feature" where we correct accounts, sounds like an unnecessary complication that we should remove.
If we get a bad account, we shouldn't try to fix it, we should just return an error.
Where does that autofixing happen?

[thsfs]

The "feature" where we correct accounts, sounds like an unnecessary complication that we should remove. If we get a bad account, we shouldn't try to fix it, we should just return an error. Where does that autofixing happen?

In the account_impl (...) function, if you look at the code segment:

...
		if (result.decode_account (account_text))
		{
			ec = ec_a;
		}
...

The decode_account function returns a corrected number.

But, in anyway, this PR is handling it correctly by using the requested account number (account_from_request) instead of the corrected one.

[thsfs]

Added a new unit test for it.