Enable customizing CreatedAt and UpdatedAt touching

[zepatrik]

Description

To enforce consistent timestamps between databases and service instances, we want to store all timestamps in UTC. However it is not possible to enforce UTC for UpdatedAt because pop will overwrite the value with time.Now() which includes the local time zone. In contrast, CreatedAt is only set to time.Now() when it is the zero value.

Proposals

There are a view ways I can think of we can allow this kind of customization of time stamps.

1. Allow setting the nowFunc. This will enable globally defining the behavior.

2. Customizing model based. Add an interface a model can implement to touchUpdatedAt and touchCreatedAt, something like

type UpdatedAtSetter interface{
  TouchUpdatedAt()
}

1. Both of above options.

I'm happy to implement this if you tell me the approach you like. I personally would prefer setting the nowFunc globally.