Always remove disconnected peers with no channels

Always remove disconnected peers with no channels

When a peer disconnects but still has channels, the peer's `peer_state`
entry in the `per_peer_state` is not removed by the `peer_disconnected`
function. If the channels with that peer is later closed while still
being disconnected (i.e. force closed), we therefore need to remove the
peer from `peer_state` separately.

To remove the peers separately, we push such peers to a separate HashSet
that holds peers awaiting removal, and remove the peers on a timer to
limit the negative effects on parallelism as much as possible.