objecter: _recalc_op_target() doesn't hand off op

Instead of handing op to target session, it just sets op->session to
NULL, we let the caller do the work later.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objecter: simplify _session_op_validate()

It's now not going to promote the rwlock anymore.

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