Purpose:

Adds write tracking to the core VFD, which can be configured
via the H5Pset/get_core_write_tracking() API call. When enabled
and writing to the backing store is enabled, this feature will
track writes and only write out the changed bytes on flush/close.

Tested on:
    32-bit LE linux (jam) w/ Fortran and C++
    64-bit BE linux (ostrich)
    64-bit Darwin 12.5.0 (kite)

All were tested using the core VFD w/ paging on (core_paged
in the VFD list). Make check-vfd was also tested on jam.


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24993 dab4d1a6-ed17-0410-a064-d1ae371a2980