[lib] Store expirationTime instead of cacheInsertionTime in ENSCache

Summary: This will allow us to vary the cache timeout for different kinds of values. I want to make failed fetches (due to network issues, not due to nonexistant ENS name) expire more quickly than successful ones.

Test Plan: Playing around with ENS on native and web

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6550