Thread-safety in executor initialization

[younes-io]

ockam/implementations/rust/ockam/ockam_executor/src/executor.rs

Line 16 in 6313b1c

pub fn current() -> &'static Executor<'static> {

Maybe we could do something like this..

pub fn current() -> &'static Executor<'static> {
    static ONCE: Once = Once::new();
    static mut EXECUTOR: MaybeUninit<Executor> = MaybeUninit::uninit();

    unsafe {
        ONCE.call_once(|| {
            EXECUTOR.as_mut_ptr().write(Executor::new());
        });
        &*EXECUTOR.as_ptr()
    }
}

Since I'm not sure how the executor is used elsewhere in the codebase, I'll let the people familiar with the codebase make sure the implementation is comprehensive..