Proposer: update fee payer check to account for multiple proposers

[MaxResnick]

We need to update the fee payer check to account for the fact that there are multiple proposers.

// for normal accounts
- account.lamports() >= fee
+ account.lamports() >= NUM_PROPOSERS * fee

// for nonce accounts
- account.lamports() >= fee + rent.minimum_balance(NonceState::size())
+ account.lamports() >= NUM_PROPOSERS * fee + rent.minimum_balance(NonceState::size())

We should also keep track of the fee payers we have already checked and keep a running tally of how much of the available sol in the fee payer balance has already been accounted for in fee payer checks this slot to further prevent DOS.