io_uring_context should use std::atomic<std:: thread::id> instead of thread_local to determine if currently on the I/O thread

[lewissbaker]

This should avoid the use of thread_local but should also allow us to detect whether a context's run() method is executing regardless of whether there is a nested io_uring_context::run() method running inside a task executed by the outer one.

[janondrusek]

@lewissbaker is currentThreadContext what you meant?