[leo_mq] Cache the result of leo_backend_db_api:status to reduce call times

[mocchira]

Now we have two code blocks to retrieve the number of records stored in leo_backend_db.

• https://github.com/leo-project/leo_mq/blob/1.4.18/src/leo_mq_api.erl#L160-L167
• https://github.com/leo-project/leo_mq/blob/1.4.18/src/leo_mq_api.erl#L253-L268

As the latter is ensured to call regularly, if we get the number and store it into the memory through leo_misc:set_env at this timing, we no longer need to call leo_backend_db_api:status at the former and instead all we have to do is just referencing the memory through leo_misc:get_env.