Estimate startup time and display in MOTD

[saminitz]

Hi there,

I had the idea to measure the time the server needs to start and then use this information on the next startup to show a countdown on the MOTD to inform the users about the time they have to roughly wait.
This is especially useful for large modpack servers which can sometimes take more than 5 minutes to startup.

Sadly I don't know rust and can't implement it myself.

[253488655422]

@saminitz I had the same thought because many don't know how to use it on our server