Add complete handler in startGraphQLSubscription

[sujeetsr]

Handle subscription completion, for the case of finite subscriptions
(apollographql/subscriptions-transport-ws#471)
@hwillson - Please review

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@sujeetsr: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[sujeetsr]

Sample client and server app to test this: https://github.com/sujeetsr/subscriptions-example

[hwillson]

This is great @sujeetsr - thanks very much!