Add nats processor that uses request reply

[hooksie1]

This is related to #2501. It adds a processor for NATS request/reply similar to how the HTTP processor works.

tagging @codegangsta

[Jeffail]

Hey @hooksie1, is this implementing similar behaviour as https://github.com/benthosdev/benthos/pull/2354?

[hooksie1]

Ah yeah looks like it. I think neither me nor @codegangsta nor @mihaitodor realized that one existed. I can close in favor of that PR if that's what's wanted.

[mihaitodor]

Oops, my bad. Completely forgot there was another PR that's similar. Sorry about that!

[hooksie1]

No it's fine. I can move the testing over to that PR since it doesn't have any. Let me know what you want me to do. The other might need updated since it's a few months old

[Jeffail]

@hooksie1 yeah I've merged the original PR, it'd be good to get some tests and general improvements on top of it, are you happy to rebase and adapt your PR?

[hooksie1]

Yeah I can do that.

[hooksie1]

ok, I think I got it. I had to modify the request reply processor a tiny bit. The way it was previously, I was getting a result store not found within batch context error from the processor. I switched from using that convert message function to just copying the message and then returning that in the messageBatch. Let me know if that's wrong, but I couldn't get it to work the other way. Fwiw this is the config I was using:

input:
  http_server:
    path: /
    allowed_verbs:
      - POST
    timeout: 5s
pipeline:
  processors:
    - switch:
      - check: '@http_server_request_path == "/test"'
        processors:
          - mapping: |
              root = json()
              root.title = "test"
          - nats_request_reply:
              urls: ["nats://localhost:4222"]
              subject: "test.thing"
output:
  sync_response: {}

[hooksie1]

@Jeffail let me know if you need me to do anything else!