Create a dedicated actor for DiskIO

[dimitribouniol]

This should have a static method that takes a url, and returns a shared actor for the volume the URL resides on. This way, all synchronous IO can be isolated to a single actor. Additionally, when running with the 5.9 runtime, we should use a custom executor to grab a new thread for that IO rather than use the swift concurrency pool, since it would eat up a slot otherwise.