Function wal_fetch_admission 

pub fn wal_fetch_admission(
    authenticated_node: Option<u64>,
    claimed_node: u64,
    local_node: u64,
    voters: &[u64],
    request_epoch: i32,
    leader_epoch: i32,
) -> WalFetchAdmission

Authorize a diskless WAL Fetch and classify its leader epoch.

Authorization deliberately precedes epoch classification, so an unauthenticated caller learns no placement epoch.