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
osd: initial COPY_FROM (not viable for large objects)

Initial pass at COPY_FROM implementation.  This uses COPY_GET to read an
object from another OSD and write it locally.  It chunks the read but
accumulates it all in-memory and commits it at once, so it is only suitable
for smaller objects.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_rados: test COPY_FROM

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