Create() uses different timestamps for default created and updated times

[mpontillo]

Description

When inserting a large number of records, I expected that the created_at and updated_at fields would always be the same. (I wanted to be able to tell whether or not a record has been updated by checking for equality between created_at and updated_at.)

Steps to Reproduce the Problem

Insert a large number of records using Create() and its default values for created_at and updated_at.

Expected Behavior

All records should have the same timestamp.

Actual Behavior

Some records have different timestamps.

When using MySQL DDL such as the following to increase the precision, every timestamp is slightly different:

`created_at` timestamp(6) NOT NULL