hdf5_msb

Fixes for issues #1 and #2 listed in John's email dated Dec 7 2018:
(1) Assertion failure in the vfd_swmr test
(2) Reader error in the vfd swmr concurrent tests
Also fixes for:
(a) Use H5MV_alloc() to allocate space for md_pages_reserved when creating the metadata file in H5F__vfd_swmr_init()
(b) Remove a multi-page (when vfd_swmr_writer is true) from the page buffer in H5MF_xfree()