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/gossipsub/src/behaviour.rs

@@ -2927,6 +2927,7 @@ where
         peer_id: &PeerId,
         connection_id: &ConnectionId,
         endpoint: &ConnectedPoint,
+        _: Option<&Vec<Multiaddr>>,
     ) {
         // Check if the peer is an outbound peer
         if let ConnectedPoint::Dialer { .. } = endpoint {