Support daemon and pidfile.

[hungrybirder]

Hello Corvus Main Developers.

I have implemented daemon feature in my branch called deamon.

Please check.

If it's OK, Please merge.

Thank you.
Best wishes!

[doyoubi]

Hi, thanks for this feature.
But now we have some good process managing tools like supervisord and systemd.
I think we don't need to build it ourselves.

[hungrybirder]

Someone can also use supervisord or systemd by adding this feature, no conflicts.
In my workspace, I don't wanna use supervisord ro systemd.
I write some scripts like corvus_install, corvus_start, corvus_stop, etc.. for managing multi corvus instances.
But it's fine~,I respect your choice.
I'll keep watching the amazing corvus project.
Thank you.
^_^

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