rgw/dedup: split-head mechanism

rgw/dedup: split-head mechanism
Split head object into 2 objects - one with attributes and no data and
a new tail-object with only data.
The new-tail object will be deduped (unlike the head objects which can't
be dedup)
We will split head for objects with size 16MB or less

A few extra improvemnts:
Skip objects created by server-side-copy
Use reftag for comp-swap instead of manifest
Skip shared-manifest objects after readint attributes
Made max_obj_size_for_split and min_obj_size_for_dedup config value in
rgw.yaml.in

refined test: validate size after dedup
TBD: add rados ls -l to report object size on-bulk to speedup the process
improved tests - verify refcount are working, validate objects, remove
duplicates and then verify the last remaining object making sure it was
not deleted

Signed-off-by: Gabriel BenHanokh <gbenhano@redhat.com>