hdf5_ep

Omnibus checkin for several relatively minor modifications:

1) Added code test/page_buffer.c to verify that page buffering is
   disabled in parallel builds.

2) Added code to test/cache_image.c to verify correct interaction
   between evict on close and cache image -- in particular management
   of a file containing a cache image containing dirty metadata that
   has been opened R/O.  Also fix for the bug exposed.

3) Added code to testpar/t_cache_image.c to verify expected procedure
   for reading cache images, and also supporting stats collection code
   needed for the test.

4) Repair of an overactive sanity check in H5C__reconstruct_cache_contents().

5) Other minor tidies in passing.

Tested serial and parallel, debug and production on Jelly.