Fix RArrowDS to support rewinding
Fix RArrowDS to support rewinding

Without this it currently works fine because RDataFrame does not rewind a DataSource while doing a run on it. However the new RCombinedDS (a RDataSource which iterates on the cartesian product of two other datasources) which I just implemented exposes the issue  because the above mentioned condition is not true anymore. This corrects the bug by effectively moving the cached cursor.
olifre/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root