* client/authority-discovery: Remove sentry node logic
The notion of sentry nodes has been deprecated (see [1] for details).
This commit removes support for sentry nodes in the
`client/authority-discovery` module.
While removing `Role::Sentry` this commit also introduces
`Role::Discover`, allowing a node to discover addresses of authorities
without publishing ones own addresses. This will be needed in Polkadot
for collator nodes.
[1] https://github.com/paritytech/substrate/issues/6845
* client/authority-discovery/service: Improve PeerId comment