HPCC-13986 Valgrind reporting memory leaks re statistics

This would leak a scope entry every time a statistic scope is entered/exited,
potentially accumulating to a significant leak over time for thor. It is less
significant on Roxie since it is only called when running queries via workunit
queue.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

system/jlib/jstats.cpp

@@ -1316,6 +1316,7 @@ public:
     }
     virtual void endScope()
     {
+        scopes.tos().Release();
         scopes.pop();
     }
     virtual void addStatistic(StatisticKind kind, unsigned __int64 value)