More SEP10 edge cases

[msfeldstein]

The server is a signer on the account, and shouldn't be allowed to sign for the account to gain weight. The server shouldn't help a client authenticate.

There's a signer on the transaction that is not recognized as belonging to the account or the server, it should decline in that case. There's no big vulnerability here, I just think it's good practice to keep that behavior predicable and that's how stellar/go#2071 is implemented.

The account has 5 signers and 2 signers sign the transaction which meet the threshold.

The challenge transaction has no signatures.

The challenge transaction has only the server signature.

The challenge transaction has no signatures and the account also has no signers.

The challenge transaction has only the server signature and the account also has no signers.

The weights of the signers add to a number greater than 255.

A non-existent account can be auth'd with its master key.