[lib] Factor out normalization functions in ENSCache

Summary: They're already used three times each, and I'm about to introduce new usages. I think it's better for this logic to be in one place.

Test Plan: cd lib && ALCHEMY_API_KEY=... yarn test utils/ens-cache.test.js

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6426