get_vote_accounts() doesn't work with forks

[rob-solana]

Problem

when setting a vote account balance to zero in a child fork, the parent fork is also cleared

Proposed Solution

introduce a forks-oriented index for the vote accounts cache

[rob-solana]

CC @sakridge

[sakridge]

@rob-solana this seems to be the same problem we fix by filtering account.tokens == 0 in the Accounts::load* functions, seems like there can be an Accounts::get_vote_accounts which does this filtering in the same fashion, what do you think?

[rob-solana]

@sakridge, I don't think that's the issue. try uncommenting and running that test, which shows that the vote_accounts are deleted in the parent

[rob-solana]

vote_accounts probably needs to have a form that's similar to transaction_count