RGW:Multisite: Verify if the synced object is identical to source

RGW:Multisite: Verify if the synced object is identical to source

Introduce an option 'rgw_copy_verify_object' which allows the object
copied from remote cluster through multisite sync is identical to the
source object. This is done by generating the MD5 checksum of the data
being copied and compared to the ETAG stored as part of the object's
attribute.

Signed-off-by: Prasad Krishnan <prasad.krishnan@flipkart.com>
(cherry picked from commit 23fb5d7b786f1034a02c991c6a648e455b5b9c30)

Conflicts:
	src/rgw/rgw_rados.cc