Cache fixes

If stream pos is greater then earliest known key and entity hasn't changed, then entity hasn't changed

Prefill stream change caches

Make /events always return a newer token, if one exists

Fix up the /account/3pid API

Add config option for setting the trusted id servers, disabling checking the ID server in integration tests

Make /sync "better".

Fix LruCache. Make TreeCache track its own size.

Cache user filters.