[CHANGED] Generate consumer name in Subscribe() when name is not provided

[piotrpio]

This PR changes the way consumer is created in Subscribe() when not using `nats.Durable().

• Previously, client used CONSUMER.CREATE.<stream_name> subject to create ephemeral consumer, consumer name being generated on the server.
• Now, consumer name is generated in the client, so that either CONSUMER.CREATE.<stream_name>.<consumer_name> or CONSUMER.CREATE.<stream_name>.<consumer_name>.<filter_subject> is used
• Ordered consumer uses the new subjects as well

[coveralls]

Coverage: 85.209% (+0.1%) from 85.075% when pulling ff32698 on subscribe-set-consumer-name into c3cae07 on main.

[derekcollison]

Be good to have a Name() option for js.Subscribe() as well, don't think that exists.

[wallyqs]

LGTM

[piotrpio]

@derekcollison agreed, although I'd call it ConsumerName() to avoid confusion... I'll add it in a separate PR.

[derekcollison]

Thanks!