signals: No need to use atomics for Handler refcount

signals: No need to use atomics for Handler refcount

handler_ref and handler_unref_R are always called with the signal
lock held. This is obvious for handler_unref_R as it even sometimes
drops this lock, and can be verified quickly for handler_ref by looking
at all call sites.

This improves the performace about 6% on the emit-handled and the
emit-handled-generic tests.

https://bugzilla.gnome.org/show_bug.cgi?id=694253