[Feature Request] Add infrastructure for sync-barrier-sync operations in parallel HDF5

[jhendersonHDF]

In past discussions of HDF5 parallel file semantics, the notion of allowing an application to be able to perform a correct sync-barrier-sync operation was discussed. This would involve:

• Adding a new 'sync' VFD callback that performs the appropriate operation for that particular VFD
• Updating all existing VFDs to correctly handle new 'sync' callback
• Adding tests for new 'sync' VFD callback
• Adding and documenting a new 'H5Fsync' API routine that calls 'sync' VFD callback
• Adding tests for the new 'H5Fsync' API routine
• Possibly updating the existing parallel I/O tests to use new 'H5Fsync' routine appropriately