hdf5_msb

Initial checkin of page buffer re-implementation to support VFD SWMR.

Tested serial / debug on Charis and Jelly.

Two known issues:

1) New page buffer seems to expose issues in the accumulator code.

   For whatever reason, fheap with the new page buffer exposes corruption
   issues if the page buffer uses H5F__accum_read/write(), but the problems
   go away if the page buffers uses H5FD_read/write() instead.

   Need to either chase this or dis-able page bufffer in combination with
   the accumulator.

2) Encountered a number of assertion failures that are explainable by the
   free space manager code not telling the page buffer to discard pages
   when they are freed.

   Wrote code to handle this -- once the free space manager is modified,
   this code should be removed and the original assertions restored.