Cleanup VAT-Filtering

[0xzrf]

Problem

VAT filtering currently is being calculated independently inside compute_new_epoch_caches_and_rewards and update_epoch_stakes while nothing enforcing them to be same.

Summary of Changes

• Compute the VAT-filtered “distribution vote accounts” once during compute_new_epoch_caches_and_rewards, and store it on NewEpochBundle
• Thread that precomputed vote-account snapshot into update_epoch_stakes, so epoch_stakes is built from the same distribution set used for rewards.
• Changed get_top_epoch_stakes to accept an optional precomputed filtered VoteAccounts, avoiding recomputation and keeping the epoch-boundary path consistent.
• Adjust Stakes::clone_and_filter_for_vat (in runtime/src/stakes.rs) to accept the already-filtered VoteAccounts when available, so epoch-stakes construction can reuse it directly.

Additional changes

As suggested in the original issue, here few more refactoring in the code:

1. in ‎runtime/src/bank/partitioned_epoch_rewards/calculation.rs, change the code to calculate filtered vote account len from:

let filtered_vote_accounts_len = stakes
                .vote_accounts()
                .iter()
                .filter(|va| va.1.lamports() > self.minimum_vote_account_balance_for_vat())
                .count()
                .min(MAX_ALPENGLOW_VOTE_ACCOUNTS);

2. add a new function Bank::minimum_vote_account_balance_for_vat() for minimum balance calculation for vat

Closes #11645

[0xzrf]

Hey @AshwinSekar
The work on this PR is done.
Thoughts on it?