[GSS] (6.4.z) Concurrent spreadsheet table validation doesn't work properly

Assume this scenarios

1. A
   1. Create a project (say, 'proj1') and upload a spreadsheet (8000rows-spreadsheet.xls)
   2. Login as user1 with browser1
   3. Login as user2 with browser2
   4. user1 validates the spreadsheet (8000rows-spreadsheet.xls) and user2 validates the same spreadsheet (8000rows-spreadsheet.xls)
   5. empty-validation-error.png reproduced
2. B
   1. Upload another spreadsheet (8000rows-spreadsheet2.xls) to the same project 'proj1'
   2. Login as user1 with browser1
   3. Login as user2 with browser2
   4. user1 validates the first spreadsheet (8000rows-spreadsheet.xls) and user2 validates the second spreadsheet (8000rows-spreadsheet2.xls)
   5. empty-validation-error.png reproduced
3. C
   1. Upload another spreadsheet (8000rows-spreadsheet3.xls) to another project (say, 'proj2')
   2. Login as user1 with browser1
   3. Login as user2 with browser2
   4. user1 validates the first spreadsheet (8000rows-spreadsheet.xls) in 'proj1' and user2 validates the third spreadsheet (8000rows-spreadsheet3.xls) in 'proj2'
   5. Doesn't reproduce the empty-validation-error.png

So, I guess we need to limit to only one validation per project (= GAV) at the same time. It should be limited at backend service.