sync: add Barrier primitive

[jonhoo]

This adds Barrier to tokio-sync, which is an asynchronous alternative to std::sync::Barrier. It is a synchronization primitive that allows multiple futures to "rendezvous" at certain points in their execution. An example execution might look like this (included in the docs):

use std::sync::Arc;
use tokio_sync::Barrier;
use futures_util::future::join_all;

let mut handles = Vec::with_capacity(10);
let barrier = Arc::new(Barrier::new(10));
for _ in 0..10 {
    let c = barrier.clone();
    // The same messages will be printed together.
    // You will NOT see any interleaving.
    handles.push(async move {
        println!("before wait");
        let wr = c.wait().await;
        println!("after wait");
        wr
    });
}
// Will not resolve until all "before wait" messages have been printed
let wrs = join_all(handles).await;
// Exactly one barrier will resolve as the "leader"
assert_eq!(wrs.into_iter().filter(|wr| wr.is_leader()).count(), 1);

[carllerche]

LGTM modulo the n == 0 question.

Thanks 👍