Use Ecto `naive_datetime_usec`

[SimonLab]

linked to: https://github.com/dwyl/phoenix-ecto-append-only-log-example/issues/14#issuecomment-440491063

To keep using microseconds Ecto provide the naive_datetime_usec type for your configuration.

From the documentation https://github.com/dwyl/alog#repo the setup of the Repo configuration is done on the application using Alog (not on Alog itself is that correct @Danwhy ?)

So we might just need to add a section in the Readme to let the users now how to add naive_datetime_usec in their repo configuration