Demonstrate IP blocking in example

This commit adds a new --block option to the server example to
illustate in a simplified way the general structure one would use to
implement IP address blocking with the new accept/reject/retry API.

For example:

    cargo run --example server ./ --listen 127.0.0.1:4433 --stateless-retry --block 127.0.0.1:8065
    cargo run --example client https://127.0.0.1:4433/Cargo.toml --host localhost --bind 127.0.0.1:8065

One thing to note is that that example places the reject condition
before the retry condition. This expends slightly less effort rejecting
connections, but does create a blocked IP address oracle for an attacker
who can do address spoofing.