[Issue 468] Add Support for NonDurable subscriptions

[dinghram]

Fixes #468

Motivation

This change allows the Go Pulsar Client to create consumers with non-durable subscriptions. We use Pulsar in an enterprise level application that processes large amounts of data throughput. We are adding a component that creates a consumer on that throughput, and it is very important that the throughput is not affected by this component. If the component creates a durable subscription, then goes down for whatever reason, the backlog caused by a durable subscription could bring our enterprise application down. Allowing a consumer to be non-durable alleviates this problem.

It has been suggested that we could use a Pulsar Reader instead of a Consumer, but then we cannot easily spread the load of messages across multiple pods, as each pod would not know which messages the other pod had processed. It has also been suggested that we could use "aggressive subscription/topic removal policies" to achieve this, but our enterprise level application has the policies already defined and changing them could have other, unintended side-effects.

This change brings a feature to the Go Pulsar Client which is already supported by the Java client.

Modifications

This change exposes the SubscriptionMode outside the pulsar package, and adds SubscriptionMode as an option in ConsumerOptions. The default remains "Durable".

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• *Added test to ensure non-durable subscription is removed when last consumer is removed.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API: yes (added SubscriptionMode option to ConsumerOptions struct )
• The schema: don't know
• The default values of configurations: no
• The wire protocol: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? GoDocs
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[shibd]

Nice work!