timeout: add support for completion_count in timeout call

[bikallem]

This PR adds support for specifying 'completion (cqe) count' when issuing timeout sqe to uring. By specifying completion_count, timeout sqe returns 0 - success - when the specified number of completion_count cqes complete.

The parameter is an optional type with default value of 0 to make it backwards compatible with earlier release.

I have also made a performance improvement to set_timespec c stub function by reducing the need to allocate int64.

submit_timeout c stub is also implemented in 2 c stubs since it has exceeded 5 parameters limit of OCaml stub functions.

[haesbaert]

I don't get why intnat completion_count, what's wrong with v_completion_count and Int_val(v_completion_count) ?
Also, I'd condense v_clock and v_absolute int a v_flags which is a list for each bit, that saves the extra parameter and you don't need the native/byte stuff.
The rest of the code is not breaking line on the function arguments so I wouldn't as well.
(edited: because of the pornographic amount of typos)