Minimise features used by RPC servers, support IPv6 CORS

[teor2345]

This PR makes two changes to RPC servers:

• add IPv6 localhost address to default CORS rules: sometimes localhost will resolve to IPv4, IPv6, or both, depending on the local C library and DNS server
• minimise features used by RPC server crates: I tested this by compiling each RPC server crate by itself
  • this also requires a fix to the gateway tokio features

Code contributor checklist:

• [x] I have read, understood and followed contributing guide