Fix the timestamp string conversion bug
Add one more test case for timestamp string conversion
Add more tests, optimize the code
Add one more test case, find a crashing problem if using || operator for since_epoch function
Switch timestamp from timestampType
Add a clear test case with comments. Return a different error
Forget to commit this updated test case
Merge pull request #625 from VoltDB/ENG-timestampConversion-bug

Fix the timestamp string conversion bug-ENG-4935