libp2p-rendezvous: Unlimited namespace registrations per peer enables OOM DoS on rendezvous servers

libp2p-rust is the official rust language Implementation of the libp2p networking stack. Prior to 0.17.1, libp2p-rendezvous server has no limit on how many namespaces a single peer can register. A malicious peer can just keep registering unique namespaces in a loop and the server happily accepts every single one allocating memory for each registration with no pushback. Keep doing this long enough (or with multiple sybil peers) and the server process gets OOM killed. This vulnerability is fixed in 0.17.1.