More correct storage format for timestamps. Fix ROOT-5331.

Solution attached as patch:
- SetTimestamp() writes timestamps of the format "2013-06-30 05:11:01.021"
  to the database and communicates the 3-digit precision (i.e., milliseconds)
  to the user in its comment. 3 digit precision is also what the strftime /
  datetime methods of sqlite use, all other digits are ignored by those methods.
- GetTimestamp() reads such timestamps, also limiting the fraction-precision
  to 3 digits and communicating that to the user from within the comment.

Patch by Oliver Freyermuth.