WIP: IO support using io_uring

Add AsyncLibrary providing IO operations which avoid the thread-blocking anomaly using the linux io_uring API. This MR makes !4 (closed) obsolete.

Fixes #2 (closed).

TODO:

• Use a dedicated thread instead of the main thread maybe?
• Overhaul SimpleDiskAndNetworkTest
• Find and fix the memory leak
• be competetive with burak's epoll implementation. Use !54 (merged) for a high performance IO implementation
• Make API really asynchronous and return a future
• Handle IO requests issued after IO subsystem is terminated
• figure out how to gracefully shutdown
• Don't unblock fibers on partial IO completion (what IO functions are feasable, recv(..., MSG_WAITALL))
• Document exported functions
• introduce timeouts to prevent denial-of-service by opening more connections than there are entries in the io_uring This is nonsense and not how io_uring works.