Allow cancellation of topic map go routine

[alexellis]

Allow cancellation of topic map go routine

Expected Behaviour

Once the topic map is being built in its own go routine, there should be some way to cancel / pause that work.

Why? Perhaps it needs to back-off for some reason, or the broker isn't available. Perhaps it should pause and restart at a later time.

Current Behaviour

Once started, there is no way to stop the topic map from being rebuilt.

Possible Solution

Introduce a Context or cancellation channel to the SDK.

The map is rebuilt in this go routine using a timer:

https://github.com/openfaas/connector-sdk/blob/master/types/controller.go#L141

Then each result is published to the subscribers via this Go routine:

https://github.com/openfaas/connector-sdk/blob/master/types/controller.go#L102

Steps to Reproduce (for bugs)

1. Setup the controller
2. Observe that it's building a map from the gateway
3. Call cancel with the new API
4. See that it stops building the map

Context

It's a best practice for Go APIs to provide cancellation.

Users have requested this in the past.