litep2p/peerset: Do not disconnect all peers on `SetReservedPeers` command (#6016)

Previously, when receiving the `SetReservedPeers { reserved }` all peers
not in the `reserved` set were removed.

This is incorrect, the intention of `SetReservedPeers` is to change the
active set of reserved peers and disconnect previously reserved peers
not in the new set.

While at it, have added a few other improvements to make the peerset
more robust:
- `SetReservedPeers`: does not disconnect all peers
- `SetReservedPeers`: if a reserved peer is no longer reserved, the
peerset tries to move the peers to the regular set if the slots allow
this move. This ensures the (now regular) peer counts towards slot
allocation.
- every 1 seconds: If we don't have enough connect peers, add the
reserved peers to the list that the peerstore ignores. Reserved peers
are already connected and the peerstore might return otherwise a
reserved peer


### Next Steps

- [x] More testing

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>