Add default implementation of ApolloCache#gc

[justinwaite]

Hello there,

Just a quick PR based on the discussion over at #5966

Please try not to be too impressed by my 4 lines of code.

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]

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

[hwillson]

Haha - 4 lines of perfection @justinwaite 😂. Thanks!