hdf5_der

[svn-r28198] Modified the metadata cache index maintenance related macros to maintain
a doubly linked list of all entries in the cache so as to allow us to 
avoid scanning the hash table.

Converted all non tagging related hash table scans to scan the index list 
instead of the hash table -- which resulted in a considerable speedup in
some cases.

Also partial code tidy.

Tested on Mercury (debug / serial & parallel) and Jam (debug / serial).
Parallel build still fails on configure on Jam.