Add vote instruction to adjust the commission

[mvines]

Problem

The commission on a vote account is fixed at creation time:

solana/programs/vote_api/src/vote_instruction.rs

Line 20 in ec5cca4

InitializeAccount(Pubkey, u32),

but if a validator is not attracting any stakers, the commission they set may be uncompetitive and they may want/need to lower it. There's no wallet command or even vote_api to do this.

Proposed Solution

1. Add a new VoteInstruction to modify the commission
2. Expose (1) as a new wallet command.

[mvines]

Probably need to think about some of the implications of a rapidly changing commission. eg:

1. Validator sets a very low commission and attracts a large set of stakers
2. Validator then bumps their commission up to 100% and rakes in the lamports until the stakers notice.

[rob-solana]

my recommendation is that we not support commission adjustments unless no rewards payments are due, or that we only support adjustments via opening a new vote account

a valildator could also bump commission to 100% and immediately run rewards redemption requests...

[mvines]

Hmm, yeah I guess a new vote account could just be created with the new commission rate. That's probably better

[rob-solana]

there's no vote_account redemption instruction...