v0.6.4: Upgrade lapin to v4; adapt AMQP runtime API

Bump lapin to v4 and remove the external tokio-executor-trait / tokio-reactor-trait glue. Use lapin's default_runtime() (owned runtime) for connections, adjust API uses to accept Into<String> where required, and construct OwnedIdentity::PKCS12 variant correctly. Update tests to match the new lapin APIs. Update Cargo.toml comment and refresh Cargo.lock with dependency version changes and transitive updates.