hdf5_der

Merge of svn trunk revisions 30102:30112 into the cache_image branch
and resolution of the resulting conflicts.

Many issues and to be dealt with, but the major ones were:

1) Removal of the notion of flush dependency height.  Since the
   cache image depends on this to order entries in the image, it
   was necessary to re-implement a form of this.

2) Addition of support for multiple flush dependency parents.
   Adding support for this required a change in the on disk image
   of entries in the cache image, changes to the image creation and
   load code, and to the management of pre-fetched entries.

3) A more subtle issue that I didn't catch immediately was Quincey's
   change in the definition of flush dependencies.  Before the change,
   a flush dependency parent could be flushed if and only if all its
   flush dependency decendants were clean.  After the change, a flush
   dependency parent can be flushed if all its immediate flush
   dependency children are clean, regardless of any other ditry
   flush dependency desendents.  In addition to changes to the
   serialization code, this required reworking of sanity checking
   code in both the cache and the fractal heap.

Changes tested on Jelly serial, serial/check-vfd, and parallel in
both debug and production builds.