Oplog Collection
    Purpose
        (‘an operations log’, entry for every write op, repl uses it, rollback, recovery, etc.)
    Oplog Visibility
        Oplog ‘Holes’
            (because parallel writes)
        Oplog Read Timestamp
            (only used for forward cursors, backwards skips)
    Oplog Truncation
        (capped collection or WT oplog stones)
        (special timestamps we will not truncate with WT -- for which we delay truncation)
        (new min oplog time retention, helps not fall off of the oplog)
        (oplog durability considerations across nodes)