core/: Concurrent dial attempts (#2248)

Concurrently dial address candidates within a single dial attempt.

Main motivation for this feature is to increase success rate on hole punching
(see https://github.com/libp2p/rust-libp2p/issues/1896#issuecomment-885894496
for details). Though, as a nice side effect, as one would expect, it does
improve connection establishment time.

Cleanups and fixes done along the way:

- Merge `pool.rs` and `manager.rs`.

- Instead of manually implementing state machines in `task.rs` use
  `async/await`.

- Fix bug where `NetworkBehaviour::inject_connection_closed` is called without a
  previous `NetworkBehaviour::inject_connection_established` (see
  https://github.com/libp2p/rust-libp2p/issues/2242).

- Return handler to behaviour on incoming connection limit error. Missed in
  https://github.com/libp2p/rust-libp2p/issues/2242.

protocols/rendezvous/examples/discover.rs

@@ -76,17 +76,6 @@ async fn main() {
                             rendezvous_point,
                         );
                     }
-                    SwarmEvent::UnreachableAddr { error, address, .. }
-                    | SwarmEvent::UnknownPeerUnreachableAddr { error, address, .. }
-                        if address == rendezvous_point_address =>
-                    {
-                        log::error!(
-                            "Failed to connect to rendezvous point at {}: {}",
-                            address,
-                            error
-                        );
-                        return;
-                    }
                     SwarmEvent::Behaviour(MyEvent::Rendezvous(rendezvous::client::Event::Discovered {
                         registrations,
                         cookie: new_cookie,