objecter, librados: add COPY_FROM operation

objecter, librados: add COPY_FROM operation

This operation will copy an entire object (data, attrs, omap)
atomically.  If the src_version does not match the source object, or
the source object is updated while the copy is in progress, we will
fail with a suitable error code.  By atomic we mean that it will either
successfully copy the entire object in its entirety or it will fail (and
require no cleanup).

Add to C++ librados API only for now.

Signed-off-by: Sage Weil <sage@inktank.com>
Conflicts:

	src/include/ceph_strings.cc
	src/include/rados.h
	src/osd/osd_types.cc