* reimplement timecache for sane and performant behaviour
* remove seenMessagesMx, take advantage of new tc api
* fix timecache tests
* fix typo
* store expiry, don't make life difficult
* refactor common background sweep procedure for both impls
* add godocs to TimeCache
* feat: expire messages from the cache based on last seen time
* chore: minor renaming
* fix: messages should not be found after expiration
* chore: editorial
* fix: use new time cache strategy consistently
* fix: default to old time cache and add todo for background gc
Marco Munizaga