Remove `Transport::Listener: Stream`. Instead require the Transport
itself to implement a stream-like API with `Transport::poll`.

In case of multiple listeners, transports are now required to handle
the multiple listener streams themselves internally.

Remove ListenersStream, instead poll the boxed transport directly
in the `Swarm` for `TransportEvent`s (which replace the former
`ListenersEvent`s).

Add new struct `GenTcpTransport` as wrapper for `GenTcpConfig` to manage
multiple listener streams.
This is essentially the old ListenerStream logic from swarm/connection.

Adapt majority of helper transports to the new Transport trait.
For most transports this just removes the extra *Listener type and
instead implements that logic in `Transport::poll`.

To adapt the `Boxed` transport the restriction had to be added that
transport is `Unpin`.
TODO: check if we can solve polling `Boxed` without the inner Transport
being unpin.

…transport-trait

With `Transport` becoming non-Clone and having `&mut` self receivers,
the `Sync` requirement no longer makes any sense and we can thus
remove it.

With PR libp2p#2667 the `Sync` trait bound for transport::Boxed is removed.
If a tcp transport should be polled as a stream we can now do this via
`TcpTransport::new(..)::boxed` and do not need a separate impl of
`Stream` for it.

Return a ListenerId in Transport::listen_on instead of getting it
injected from the outside.
Namespace ListenerIds with the Transport's TypeId to avoid clashing IDs
when transport generate their ListenerIds independenlty.

…transport-trait

…transport-trait

…transport-trait

…transport-trait

Co-authored-by: Max Inden <mail@max-inden.de>

Co-authored-by: Max Inden <mail@max-inden.de>

…transport-trait

…transport-trait