Added BackOff helper method to set backoff through subOpts

[mfaizanse]

• Added BackOff helper method to set backoff through subOpts

[coveralls]

Coverage increased (+0.2%) to 85.307% when pulling c47689c on mfaizanse:backoff_sub_opt into e0e03e3 on nats-io:main.

[derekcollison]

What is the goal of this change? For true push I am worried that this could collide with flow (or lack thereof) and cause memory bloat in the client app.

Could be useful maybe for when you do a pull request however but best to understand what the exact intent is here.

[mfaizanse]

The NAK delay and backOff is implemented in these PRs: nats-server and nats-go.

Currently, in the API for to create a consumer, we can set the BackOff but if we create a subscriber then we don't have the Opts method to set the BackOff. The goal of this PR is to enable the user to set the BackOff when creating through a subscriber like this:

// Create a subscriber
	_, err = js.Subscribe(
		subject,
		callback,
		nats.Durable("consumer1"),
		nats.ManualAck(),
		nats.AckExplicit(),
		nats.BackOff([]time.Duration{50 * time.Millisecond, 250 * time.Millisecond})
	)

[kozlovic]

Would add a test and either remove from example or "fix" it as described in the comments.

@derekcollison We (I think I did) added the features (Nak and Backoff) but I simply forgot to add the option. So I think @mfaizanse is just fixing my omission here :-)

[kozlovic]

This complies, but would actually not "work" because you would need to set the MaxDeliver to at least 2 (or more), but also add nats.ManualAck() because otherwise the callback would auto-ack anyway, so it is unlikely that there would be redeliveries...

So instead (or at least in addition to) having this here, I would update, say TestJetStreamSubscribe_AckPolicy test to use the nats.BackOff() new option. In this test (at the end of it), we use backoff but with the AddConsumer directly.

[kozlovic]

LGTM

[kozlovic]

@derekcollison I have approved this PR, let me know if you still have concerns: we have added the BackOff list consumer configuration in the client, so not sure why is that an issue to make it an option for the js.Subscribe() calls...