Avoid generating client IDs in substrate

[slawo]

https://github.com/uw-labs/substrate/blob/67626b38e3a2382235b2307e2b82d50f297cec6b/natsstreaming/nats_streaming.go#L173

This type of convenience can introduce unwanted behaviour upon re-connections and should not be silently ignored.

It is preferable for the client to generate its own random id when the user is aware of the consequences.