Change to store swapmeta: addition of a new type

Part of my COSS rewrite required a little 'help' from the store system
to hand over a little more metadata. This metadata is then used during
the COSS store rebuild.

I've added a new metadata type which records the object size if known.
Since its always known for COSS stores it means the object size is
written in as part of the metadata - and therefore allows COSS to 'know'
what the object size is when reading and rebuilding. The header isn't
required for swapin; nor is it checked at all in the other stores.
(It could be used as a further consistency check for aufs/ufs/diskd
during rebuild.)

I'd like to eventually add a small header to each COSS object to
include the total object size (data+metadata) and object "written"
time to help the rebuild process even further; but I've tried
to keep the actual storage layout unchanged in this first pass.

I've attached a patch for comment.

Thanks,

Adrian