Reconnection is now handed by cancelling the connCtx context. This is a
lot cleaner and the jetstream client package honors context
cancellation.

Saving of last time_us is also simplified now. We only save (update,
rather) after the last seen event by incrementing the event's time_us by
1 (this is ok since it's a monotonic clock). We pick up from here upon
reconnect and don't save last time_us for any other reason.

Lastly, outside of first boot, we should only ever use UpdateLastTimeUs.