hdf5_msb

In H5PB__write_meta(), extend a single-page entry when overwriting it
with multiple pages.  Update statistics to maintain consistency.

Refactor a bit: in H5PB__write_meta(), move code that's in both the if-
and else- branch to either before the if-else or after and de-duplicate.

In H5PB_vfd_swmr__update_index(), always update the length of a
shadow-index entry to the current size of its corresponding page-table
entry.

In H5PB_vfd_swmr__update_index(), disregard shadow-index entries that
await garbage collection.