Support durable queue subscriptions.

[bmcustodio]

Description

This PR adds support for durable queue subscriptions while maintaing the implementation backwards-compatibility.

Motivation and Context

• [x] I have raised an issue to propose this change (required)

75.

How Has This Been Tested?

Using the steps described in #75 while using the bmcstdio/openfaas-nats-queue-worker:nats-durable-queue-subscription image.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[bmcustodio]

@alexellis can we get this merged please? 🙏

[alexellis]

Thanks for the PR.

Something to note from the NATS team - durable subscriptions are not "cleaned up" when you disconnect, so you'd have to do that manually if they are named.