fix(pubsub): callback from unsubscribe after stream ends

[alanshaw]

This PR changes the unsubscribe logic to only callback on the last event we can listen for on a dropped connection.

This still does not guarantee that your topic won't appear when calling pubsub.ls in the callback to pubsub.unsubscribe - but only when talking to go-ipfs.

Adding an artificial timeout using setTimeout seems to provide enough time for go-ipfs to actually do the unsubscribe.

[daviddias]

Now seems that a new fail on DHT is happening:

  1) .dht

       "before all" hook:

     Error: Uncaught AssertionError: expected [Error: Unexpected end of JSON input] to not exist (webpack:///node_modules/chai/lib/chai/assertion.js:141:0 <- node_modules/aegir/src/config/karma-webpack-bundle.js:93052)