Add support for EXTEND_TIMEOUT_USEC to systemd notification support

[behrmann]

Is your feature request related to a problem? Please describe.

Using puma in GitLab and running puma as a systemd service with type notify I ran into startup issues because preloading the application took longer than the default service start timeout of 90s.

Describe the solution you'd like

It would be great if puma could write EXTEND_TIMEOUT_USEC= with some integer value regularly to the notification socket before signaling READY=1 and before running into the timeout to extend its startup time.

Describe alternatives you've considered

The issue can be easily worked around on a case by case basis by giving puma more time to start in the service file, i.e. setting TimeoutStartSec= to a higher value

[nateberkopec]

Makes sense, I think we'd be willing to include this.