hdf5_msb

H5F_vfd_swmr_reader_end_of_tick(),
compares both new and old shadow indices and calls H5PB_remove_entry()
on each entry that was in the old index but is not in the new.

Ever since H5PB_remove_entry() started removing shadow index
entries, it has been possible for H5F_vfd_swmr_reader_end_of_tick()
to walk past the end of the new shadow index or even to skip entries
in the new index.  Sometimes an assertion failed when that happened.

I have restructured the code in H5F_vfd_swmr_reader_end_of_tick()
so that it compares the old and new indices, gathering a list of
removed pages, in one step.  In the next step, it processes the
list of removed pages, calling H5PB_remove_entry() on each page.
In the step after that, it notifies the metadata cache of each
removed page.  This fixes the bug I described, above.