Strategy::OnInterest lookup ContentStore
fix duplicate Interest timeout bug

Problem: `ccnpeek /v/ping/1` is unanswered if previous Interest is answered, ContentEntry is stale, but PitEntry is still kept. This may cause assert failure.
Root cause: OriginalStrategy::PropagateNewInterest assumes there's only one PitDownstreamRecord; PitUpstreamRecord->pending() is not cleared.