Start/stop multiple servers

[VDigitall]

Description

It would be highly beneficial to have the ability to start and stop multiple servers simultaneously within the pymodbus library. I saw that implementation but it was removed two years ago.

It would be good to have the possibility:

• Start a few servers
• Stop all servers at once.
• Stop each server separately

[janiversen]

You can actually already do that by making a simple function....have a look at how start*server works and collects and array of active servers.

Pull requests are always welcome if you want changes.