hdf5_msb

Simplify H5PB_vfd_swmr__update_index(), which figures in
my investigation of shadow-file freespace leaks.

Copy the tick number from the H5F_shared_t to a temporary variable
and use the temporary instead of spelling out `shared->tick_num`.

Assert that every entry on the tick list is dirty, since clean
entries will not appear on the tick list.  Mark each corresponding
shadow-index entry dirty, and set its "tick of last change" to the
current tick and "tick of last flush" to 0.  This makes the scan
for entries that do not appear on the tick list more believable.

Lower staircase in the loop that scans for shadow-index entries that
did not appear on the tick list.  Log indicies that are marked for
reclamation.