Remove dynamic dispatch from _wait/wait2

[vchuravy]

While looking at the performance off:

using Base.Iterators: partition

function axpy_per_core!(y, a, x)
    indices = eachindex(y,x)
    tasks_per_thread = 1 # customize this as needed. More tasks have more overhead, but better load balancing
    chunk_size = max(1, length(indices) ÷ (tasks_per_thread * Threads.nthreads()))
    indices_chunks = partition(indices, chunk_size)

    @sync for chunk in indices_chunks
        Threads.@spawn begin
            for i in chunk
                y[i] = muladd(a, x[i], y[i])
            end
        end
    end
end

y = rand(10_000);
x = rand(10_000);

axpy_per_core!(y, 2.0, x)

Gabrield and I noticed that a not-insignificant amount of time is spent on the dynamic dispatch.
As far as we can tell we are gurantueed that donenotify is a ThreadSynchronizer.
See task_done_hook.