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.

examples/file-sharing.rs

@@ -492,13 +492,8 @@ mod network {
                     }
                 }
                 SwarmEvent::ConnectionClosed { .. } => {}
-                SwarmEvent::UnreachableAddr {
-                    peer_id,
-                    attempts_remaining,
-                    error,
-                    ..
-                } => {
-                    if attempts_remaining == 0 {
+                SwarmEvent::OutgoingConnectionError { peer_id, error, .. } => {
+                    if let Some(peer_id) = peer_id {
                         if let Some(sender) = self.pending_dial.remove(&peer_id) {
                             let _ = sender.send(Err(Box::new(error)));
                         }