Adapt VerifyKey function behavior and create a new DecodeMemo function

[VIM-Arcange]

As of now, we have to pass a memo to the verifyKey to check the existence of a specific key in the wallet.

An app wanting to verify a key will call steem_keychain.requestVerifyKey with anything as a memo and will get: {error: "decode_error", message: "Could not verify key.", ... , success: false} meaning the requested key exists but we failed to decode.

It looks strange to manage an error to validate a successful test.

IMHO, DecodeMemo and Verify Key should be 2 separate functions.