Do not save chunks when auto saving is disabled

[PetteriM1]

This fixes #272

Currently Nukkit saves the chunks when the level is unloaded even if auto saving is disabled. This pr makes chunks to be never saved automatically if the auto saving is disabled (and if none of your plugins does the saving or you don't use the save-all command). The question is: Should disabling auto save only disable the auto save task or also the saving Nukkit does when levels are unloaded? Not saving chunks at all when the auto saving is disabled may be usefull on minigame servers where the worlds will be reseted after a match anyways.

[SupremeMortal]

I don't think this is correct. Auto-save is the task of saving the world regularly whilst it's running so in the event of a crash, the server does not lose as much data. The proposed change turns auto-save into a property more like read-only or non-persistent world which should be a separate configuration field.

[Guillaume351]

Creating a separate setting for this could be good, it’a much easier to reset mini game maps when you can just unload and reload them

[PetteriM1]

Proper API included in https://github.com/CloudburstMC/Nukkit/pull/2175