Refactor Sync and fix `initialSyncLimit` (#2587)
* Small tidy-up to sync.ts
* Convert doSync into a while loop
* Apply `initialSyncLimit` only to initial syncs
* Convert matrix-client-syncing spec to TS
* Add tests around initial sync filtering
* Switch confusing filterId field for `filter`
* Tweak doSync error control flow
* Fix error control flow intricacies
* use includes
* Add tests
* Fix some strict mode errors
* Fix more strict mode errors
* Fix some strict mode errors
