This routes all world generation requests through PaperLib, which will generate Chunks asynchronously if the server allows it (Paper does, Spigot doesn't). This means changes to which chunks are still needed, and which can be unloaded, as well; the code keeps a list of Chunks that are needed for others, and will unload them only when the target chunk has been generated. Unloads by the server itself get prevented while the chunk is needed; else the server could decide on a tick that chunk has no players nearby and needs to be unloaded.
This option was mainly added for people wanting to follow Mojang's recommendation to load all important world chunks in 1.6.3 - 1.6.4 before updating to the eventual 1.7 release to prevent errors with structures such as witch huts and nether fortresses:
https://mojang.com/2013/09/minecraft-snapshot-13w37a/
Guntram Blohm
Brokkonaut
Brettflan
Nick D
Ian Macalinao