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.
