For compatibility with versions below 4.2, we still
use the `max_inflight_size` option, but generate it
as the `max_inflight`
Previous PR: https://github.com/emqx/emqx/pull/3938
* fix(bridge_mqtt): max_inflight_size -> max_inflight; this enables emqtt.erl to use the value
* chore(bridge_mqtt): fix elvis
* fix(worker): max_inflight_batches -> max_inflight