[performance] do not assume that `== can modify call_outs

the call_out data structures in backends are protected again reentrance,
so we can safely assume that `== does never modify the call_out hash
table. this change makes backend_find_call_out significantly faster and
speeds up the corresponding call_out handling benchmark by 25%. It is
still _much_ slower than in 7.8, probably because call_outs are objects
in 7.9, which introduces quite some overhead.