Add config for chunk unload drains per tick

[Rothes]

After checking my memory dump, I found that the pending unlock chunks takes up most of my memory. I have a lot of players who love to travel around with elytra.

There's a maxAutoSaveChunksPerTick option, so I think these UnloadChunksPerTick options make sense.

After setting maxUnloadChunksPerTickFactor to 1.0 on my server, my memory comes very healthy.

[lynxplay]

Changes to moonrise need to be PRed to its repo, otherwise we'll loose them on next re-apply of the patch https://github.com/Tuinity/Moonrise

[Rothes]

Uh, we need to add the configuration entry to Paper WorldConfiguration.

Should I just pick another patch commit?

[lynxplay]

No, this is not about the chosen patch, this is about the moonrise to paper flow.

The moonrise solution to this is an addition to the PlatformHooks type, which serves as the common config interface.
That is then implemented on paper via a world configuration option.