Fix async engine reloading

[Steveplays28]

Also made the /ambient-reload command reload the engine asynchronously. Follow-up PR to #246, should avoid any resource reload (compatibility) issues in the future such as https://github.com/IMB11/Fog/issues/23.

If you spam the command/resource reload fast enough it'll keep reloading the engine with this code, though I'm not sure if that's a big issue requiring a fix currently. You'd have to check if a CompletableFuture<Void> field is already reloading the engine.