osd: keep track of current osd subop num

This can be used later to generate unique subop versions
in compound write operations.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objclass: provide new api for unique subop versioning

We need to be able to generate a unique identifier for
each subop. This can be useful e.g., if we want to keep multiple
omap data entries indexed by the same key. The unique id
is being generated by the current object id and the current
osd subop count. For write operations it's unique.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
cls_log: fixes, other adjustments

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
cls_log: unitest

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>