hdf5_der

Added preliminary cache image parallel test, and
fixes for several bugs exposed by this new test.
In particular:

    1) Cache image / R/O bug:  In R/O case, dirty entries in the cache
       image must be inserted into the metadata cache marked clean so that
       the cache will not attempt to write them on close.  However, this
       allowed the cache to evict these entries and later try to reload
       them.  Fixed by adding code to prevent the eviction of such entries.

     2) Several flush dependency / flush ordering bugs in PHDF5

     3) With the version 2 superblock, it is now possible that there
        will be more than one entry in the metadata cache marked "flush
        me last".  This broke the distributed metadata write code.  Re-wrote
        this code to address this -- new code ignores "flush me last" and
        instead observes rings.

Note that this checkin is preliminary -- need more parallel / cache image
testing, and more investigation of the bugs exposed by same.

Tested on Jelly, serial and parallel, debug and production.