Improves server ticking.

As I was lying in my bed, I came to a solution for the tick problem.
While remembering how most of the time the server is sleeping,
and how the TPS drops once costly operations are ticked,
I reviewed mentally that part of the code, Server::tick().

Then I saw it: I was setting the next execution time
using the time after it was executed as the base, but it should
have been done using the tick start time - that way if something takes
longer, the server will catch up and won't drop the TPS
until the CPU hits its limit.

I got up, got to my computer, and checked that function.
It was exactly as I saw in a near-dream state, so I fixed it
and tested things if they worked right.

Now I'm fully awake and I can't sleep anymore, so I wrote this.