Make cling::Transaction cheaper to construct. Lazy-create the container of the 
nested transactions. 

Now sizeof(cling::Transaction()) == 56.


git-svn-id: https://root.cern.ch/svn/root/trunk@49124 27541ba8-7e3a-0410-8455-c3a389f83636
Fix a misnomer.


git-svn-id: https://root.cern.ch/svn/root/trunk@49125 27541ba8-7e3a-0410-8455-c3a389f83636