objecter, librados: create Objecter::Op in two phases

(currently only in some librados operations)
First create the op, only then lock and submit so that we reduce lock
contention.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
librados: lockless get_instance_id()

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #993 from ceph/wip-librados-lock

Wip librados lock

Reviewed-by: Sage Weil <sage@inktank.com>