Server: allow revoking a client key at runtime (RemoveAllowedClient)

I run tailcat serve --allow=… on an Android box and pair phones to it by adding their node keys. Server.AddAllowedClient lets me add a key while running, but there is no way to remove one: the only way to revoke a phone is to restart serve with a shorter --allow, which also drops every other client for a few seconds until they re-meow.

Would you take a Server.RemoveAllowedClient(k) that (1) makes further meows from k be ignored and (2) drops k from the network map if it is currently connected? For the CLI, an --allow-file=PATH that is re-read (on SIGHUP, or when the file changes) would cover the same need without a control socket.

Note: This is my first time working on an Open Source Project, every remark are welcomed :)

Start by tracing Server.AddAllowedClient and the serve --allow handling to understand how allowed keys and the network map are managed. Implementing this should make later meows from a removed key ignored and remove an already connected client; verify both behaviors with the project's existing tests or runtime checks.