enhance: Use mvcc timestamp as guarantee ts if set

When MvccTimestamp is set, it could be used as guarantee timestamp
directly instead of new ts allocated by scheduler reducing the waiting
time when delegator has tsafe lag

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/proxy/task_query.go

@@ -570,6 +570,7 @@ func (t *queryTask) queryShard(ctx context.Context, nodeID int64, qn types.Query
 	retrieveReq.GetBase().TargetID = nodeID
 	if needOverrideMvcc && mvccTs > 0 {
 		retrieveReq.MvccTimestamp = mvccTs
+		retrieveReq.GuaranteeTimestamp = mvccTs
 	}
 
 	req := &querypb.QueryRequest{