hammer_tid_t
hammer_tid_t sync_low_tid; /* full history beyond this point */
hammer_tid_t sync_beg_tid; /* earliest tid w/ full history avail */
hammer_tid_t sync_end_tid; /* current synchronizatoin point */
hammer_tid_t tid; /* the snapshot TID itself (== key) */
hammer_tid_t vol0_next_tid; /* highest partially synchronized TID */