Support server-client connection validation with version checking

Currently, we can read versions of both `RouterClient` and the connected `RouterServer`, but we don't do any validation upon connecting. This is fundamentally hard because the underlying connections are done asyncrhonously with zmq. In other words, you can launch a client without any server connection, and vice versa.

One possible solution would be to add extra connection validation hooks. so that the user, at their own discretion, can do the validation before doing any further communication. Something like a `validate_server_version` from the client side. Even providing status information like `has_connected` from the client side could be useful.