Correct handling of `UnknownRequiredFeature` deserialization

Correct handling of `UnknownRequiredFeature` deserialization

Quite some time ago, `UnknownRequiredFeature` was only used when a
gossip message has a missing required feature. These days, its also
used for any required TLV which we do not understand in any
message. However, the handling of it was never updated in
`PeerManager`, leaving it printing a warning about gossip and
ignoring the message entirely.

Instead, we send a warning message and disconnect.

Closes #1236, as caught by @jkczyz.