transport-manager: Fix memory leak by bounding the number of tracked peers

[lexnv]

Currently, the transport manager never removes the state of disconnected peers.

This leads to increasing memory consumption over time, especially considering that each peer state supports up to 64 addresses.

We would still need to keep disconnected peers around for a sensible amount of time because the peer state provides us with the addresses to dial peers.

One possible solution that takes into account preserving addresses:

• extend PeerState::Disconnected with a timestamp; whenever the peer disconnects we keep track of time
• periodically (tbd N minutes 1h) remove disconnected peers who are disconnected for more than N minutes

Total number of tracked addresses

Total number of tracked peers

cc @paritytech/networking