Fixed concurrency issue in InsertScan and PosUpdateScan:
Thread A: fetch before_size
Thread B: fetch before_size
B: append_to_delta
A: append_to_delta
For A, before_size+i is not the correct way to address its new rows anymore
Merge pull request #273 from hyrise/fix-insertscan-concurrency

Fixed concurrency issue in InsertScan and PosUpdateScan: