osd/PrimaryLogPG: do not set data/omap digest blindly

osd/PrimaryLogPG: do not set data/omap digest blindly

As bluestore has bulitin csum, we generally no longer generate
object data digest for now. The consequence is that we should
handle data/omap digest more carefully to make certain ops,
such as copy_from/promote, to work properly since they heavily
relies on data digest for data transfer correctness.

Example of failure:
http://pulpito.ceph.com/xxg-2017-09-30_11:46:34-rbd-master-distro-basic-mira/1690609/

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit be078c8b7b131764caa28bc44452b8c5c2339623)

Conflicts:
	src/osd/PrimaryLogPG.cc (still have snapdir)