Enable to add listeners for clearStore

[joe-re]

ref: apollographql/apollo-feature-requests#53

Add onClearStore method to client instance for hook clearStore.
This provides us to manage some kinda application lifecycle after clearStore, certainly.

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]

@joe-re: 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/

[hwillson]

This is awesome @joe-re - thanks very much!