hdf5_ep

[svn-r2501] Added several free lists for commonly used & abused data structures.  Also
added code to allow metadata to be allocated out of a more contiguous block
("metadata aggregation") and also code for "catching" small metadata write
calls and building a buffer of the small pieces of metadata for later writing
as one, larger, block ("metadata accumulation").  These features are enabled
on a per VFL driver basis with the new VFL 'query' call and both currently
enabled for the sec2, family and stdio drivers.  The mpio VFL driver enables
only the "metadata aggregation" code, not the "metadata accumulation" code.
All the other drivers have these features turned off.