[lib] Make it possible for ENSCache to store Promises

Summary:
This is a necessary step for the next diff, where make it so we don't fetch a single result from ethers.js more than once at a time.

Depends on D6429

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/D6430