Generator rate limits are ineffective when many parallel worlds are used

[dktapps]

Issue description

It's common particularly in SkyBlock servers for many tens or hundreds of worlds to be loaded simultaneously, since every player often gets their own island.

Generator queues are currently enforced to prevent overloading the async worker pool with generation tasks. However, there is a serious problem with this mechanism: the queue size is limited per world.

This means that players flying around generating terrain independently in several parallel worlds may cause CPU overload regardless. In the case of very compute-expensive or time consuming generators, this can result in serious performance issues without obvious cause (async tasks aren't recorded for timings reports).

OS and versions

• PocketMine-MP: 3.17.0 / 4.0.0