Fix for timestamp precision in SQLAlchemy

This fix changes the SQLAlchemy models to use Decimal datatype
instead of DateTime, because MySQL truncates to the second:
http://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html

Change-Id: I9d324d1cb6867cf99e15e2ecdc566dc5f8fef536
Closes-Bug: #1215676