The bug: start() was setting lastLoadedPubkey at the beginning, so if
start() got called twice (which it was), the second call would see
isLoadedFor(pubkey) return true and skip the entire loading process,
including fetching mark-as-read reactions.
Fix: Only set lastLoadedPubkey AFTER all fetching is complete. This
ensures that concurrent start() calls don't skip the loading.
This allows kind:7 and kind:17 mark-as-read reactions to be fetched
and tracked properly.
3991bfeeb2