Add optional generics to cache manipulation methods

[mvestergaard]

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
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

This sort of falls outside of all of the checklist items above, so I checked them.

Currently, when using the read/write methods on the cache, it is very easy to make mistakes, by simply misspelling a variable for example.
This PR adds optional generic arguments to methods used to manipulate the cache.
I've made sure to add a default for all of the new generic arguments, so it should not break anything.

[apollo-cla]

@mvestergaard: 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/

[apollo-cla]

	Warnings
⚠️	There are library changes, but not tests. That's OK as long as you're refactoring existing code

Generated by 🚫 dangerJS

[hwillson]

Thanks very much @mvestergaard! We should be good to go here - LGTM!