Minor changes to persistence code
Minor changes to persistence code

Mainly, makes it closer to the multithreaded-interpreter version.

minesense/VisTrails,Nikea/VisTrails,hjanime/VisTrails,celiafish/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,minesense/VisTrails,Nikea/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,Nikea/VisTrails