hdf5_der

[svn-r28477] Checkin of fix to SWMR-95 -- bug in which two calls to 
H5Oget_info_by_name() resulted in an assertion failure
in the metadata cache.

Investigation of this bug uncovered a number of issues,
only some of which are addressed in this checkin -- 
specifically incorrect takedown of the flush dependency
between the oject header proxy and the extensible array
header, and attempts to setup this flush dependency 
repeatedly.

tested (serial/debug only) on mercury and jam