When `-- features tls` is specified for tarpc, RPC communication can
also occur over a `TlsStream<TcpStream>` instead of a `TcpStream`.
* The functional tests have been refactored to use a common set of
functions for constructing the client and server structs so that all
the tests are shared across non-tls and tls test runs.
* Update pre-push to test TLS
* The `cfg_attr` logic caused many false warnings from clippy, so for now the crate docs for TLS are not tested.
