Version 9 introduced DataStore and WolframIOLibrary_Functions in WolframLibrary.h. They don't seem to be documented.
There are some examples using these in
SystemOpen@FileNameJoin[{$InstallationDirectory, "SystemFiles", "Links", "LibraryLink", "LibraryResources", "Source"}]
The examples are in the files named as async-*.
What is this functionality for? It would be nice to have a short introduction or tutorial on using these.
Possibly related: