Fix bug where some ledgers are not being published (#281)

* The ledger close time can occasionally be a few seconds in the future,
  which causes ETL to not publish the ledger, because the age
  calculation wraps around and the age is computed as a very large
  unsigned integer. This fix rounds to zero when the age would be
  negative

src/etl/ReportingETL.h

@@ -382,6 +382,8 @@ public:
                        std::chrono::system_clock::now().time_since_epoch())
                        .count();
         auto closeTime = lastCloseTime_.time_since_epoch().count();
+        if (now < (rippleEpochStart + closeTime))
+            return 0;
         return now - (rippleEpochStart + closeTime);
     }
 };