[PATCH 0/3] update chunkd checksum verification scheme

This patchset is part of the work necessary to get ranged-GET (aka
partial GET) working.  As explained in
http://marc.info/?l=hail-devel&m=127871407125539&w=2 the current chunkd
checksum scheme does not work at all for partial retrievals, and must be
revamped.

These patches present step 1 of 4, adding a table of checksums to
chunkd's local on-disk format.

There are no protocol or API changes in this patchset, existing clients
should work fine without any changes.

Nevertheless, this will not be committed to the main branch until
partial retrieval is actually implemented.  I don't commit changes
unless they are actually neeeded.  This checksum table and sendfile
removal work is not required until partial-GET actually exists.

	Jeff