TaskManager: rename _newArchiveMap -> nextArchiveMap
TaskManager: rename _newArchiveMap -> nextArchiveMap

This isn't a member variable, so it shouldn't have a leading underscore.
Also, the old name was vague whether it was a "new ArchiveMap", or a
"map of new archives".

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui