Key preference mechanism

[jeffreyssmith2nd]

Add a mechanism for indicating a preference for which keys to use for signing a transaction.

[randytorres]

@jeffreyssmith2nd is this like owner vs active key?

[jeffreyssmith2nd]

Yes, except the idea being that you could have any number of arbitrary keys and choose which one you want to sign with. I believe @brandonfancher and @tbfleming had thoughts on how it should be implemented.

[tbfleming]

More like idea fragments :(