Go over our database update statements and exclude created_at updates

When I was working on #996 (closed), I realised that our dbmodel.Update... calls may be wrongly updating the created_at columns. It becomes apparent when you have an instance in the database and, instead of getting it before the update from the database, you create a sibling instance and attempt to use it in the update. In that case, you typically don't set the created_at value and it defaults to nil. As a result, go-pg will try to set the nil value which is prohibited due to the NOT NULL constraint.

In general, the updates should not modify this timestamp. Therefore, the created_at column should be excluded from the update statement, like this:

	result, err := tx.Model(host).WherePK().ExcludeColumn("created_at").Update()

I think we're lucky we haven't hit this issue before.