[meson] set check_anywhere_queue_while_steal automatic

We introduced the check_anywhere_queue_while_steal configuration as an optimization to get the IO completions reaped by the completer faster into the normal WSQs. But now the emper has configurations where we don't use a completer thus making this optimization useless or rather harmful.

By default automatically decide the value of check_anywhere_queue_while_steal based on the value of io_completer_behavior.