Move attachment code into couch_att

This is moves a majority of the attachment representation
into the couch_att module. This serves to isolate the
current record to allow easier in-place upgrades as well as
a place to start collecting common attachment related
functionality.

The upgrades are handled lazily to allow rollbacks to older
code if the new attachment format has not yet been required
via storage of any of the new extended attributes supported
by the fetch/store APIs. There are some caveats to this in
that the extended attributes are not enforced by couch_att
at this time so it'd be quite easy to store garbage. As the
extent of attachment concerns becomes more stable, a set of
more permanent fetch_[field]/store_[field] functions may be
added to help enforce both field types as well as common
field names and defaults (all fields will default to
undefined except for those defaults present in the orignal
record definition, which carry over automatically).

Finally, while this patch does move a lot of code to
couch_att, it hasn't refined the interfaces much. These
changes will follow in later patches to improve and
simplify the organization of attachment code. This
includes the addition of more unit tests which currently
only cover some portions of the attachment functionality
related to upgrades and field fetching & storage.
Test all windsor-merge branches