osd: explicitly create new object,snap contexts on push

osd: explicitly create new object,snap contexts on push

We specifically want to use this during recovery to avoid loading the obc
or ssc for a previous version of the object and populating the watchers.
We know we won't have any existing obc here because it is missing (old or
dne).

For the snapset context, we provide it explicitly when we recover the head
or snapset object (which we always do first).  For clones, we re-use the
existing get_snapset_context(), which will either have the ssc open or
can load it from the head/snapset object.

Signed-off-by: Sage Weil <sage@newdream.net>