Add a progressive executor to execute minor or long tasks

[AmauryCarrade]

A tool to register tasks to do (maybe a lot of tasks); these tasks are executed in “semi-background”, one every few ticks.

Examples of use:

• the wall generators (one task per column of the wall, so the game remains fluid when the wall is regenerated);
• the miscellaneous tasks executed when a player die (so there isn't any latency in small server, all this stuff is not executed in a single tick).

An idea inspired by @prokopyl, thanks!