Making `solana_vote_program` is not an option because
then vote_program's entrypoint conflicts with reward_program's
entrypoint.
This unfortunately turns the SDK into a dumping ground for all
things shared between vote_program and other programs. Better
would be to create a solana-vote-api crate similar to the
solana-rewards-api crate.
And add a new 'staker_id' VoteState member variable to offer a path to
work our way out. Update leader scheduler to use staker_id, but
continue setting it to 'from_id' for the moment.
No functional changes here.
