Fix compilation on 32-bit systems

storesession.h was declaring fields as std::atomic<uint64_t>, a type
not avaiable on 32-bit systems.  However, the values stored in these
fields were only used as input to QProgressDialog, which takes int,
not uint64_t.  So the fields could just as well be std::atomic<int>.

Also, added code to scale the progress values down if they would not
fit in an int.  (This would have been needed even if the fields were
to remain as uint64_t.)