Only update statuses/users when merging if they are newer.

Title sounds confusing, here's an explanation:

PleromaFE keeps track of all the statuses (and users) in vuex. Currently when some request finishes, all user and post data is passed through deduplication function which merges new data into existing data. That function should not update existing data if it's stale.

Stale data could easily happen with asynchronicity - JS sends requests 1 2 3 but receives 1 3 2, and parses them 3 2 1 or any order in general. To avoid stale data we should check timestamp from server's response's header (Date header) and keep it, update existing data if new data has newer timestamp that stored one.