[df] Align RDataSource cleanup routine with other sources

The call to cleanup routines was introduced in commit
0a719b4

The commit description informs it was done to avoid having setup and teardown
routines called multiple times for the same column. In single-thread processing, the concept
of one task is the processing of the entire computation graph for that run. As such, CleanupTask
is called in RunTreeReader and RunEmptySource at the end of the function. In the case of
RunDataSource, this routine is called in-between every event range processing. This commit aligns
the RunDataSource logic with the logic of the other two single-thread processing methods, i.e.
CleanupTask is now called at the end of the run.

Author: vepadulano

tree/dataframe/src/RLoopManager.cxx

@@ -737,10 +737,10 @@ void RLoopManager::RunDataSource()
 {
    assert(fDataSource != nullptr);
    DSRunRAII _{*fDataSource};
+   RCallCleanUpTask cleanup(*this);
    auto ranges = fDataSource->GetEntryRanges();
    while (!ranges.empty() && fNStopsReceived < fNChildren) {
       RDSRangeRAII __{*this, 0u, 0ull};
-      RCallCleanUpTask cleanup(*this);
       try {
          for (const auto &range : ranges) {
             const auto start = range.first;