rgw: scalable manifest object

Define a new manifest structure. The idea is that the manifest defines a
set of rules for structuring the object parts. There are a few terms to
note:
 - head: the head part of the object, which is the part that contains
   the first chunk of data. An object might not have a head (as in the
   case of multipart-part objects.
 - stripe: data portion of a single rgw object that resides on a single
   rados object.
 - part: a collection of stripes that make a contiguous part of an
   object. A regular object will only have one part (although might have
   many stripes), a multipart object might have many parts. Each part
   has a fixed stripe size, although the last stripe of a part might
   be smaller than that. Consecutive parts may be merged if their stripe
   value is the same.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
test/rgw: manifest unitest

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
cls/rgw: fix debug output

print the correct error value

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
dencoder: fix for new rgw manifest code

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1214 from ceph/wip-rgw-manifest-2

Wip rgw manifest 2
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>