Support `diesel_manage_updated_at` on SQLite #1871
There's a little bit of funkiness required to make this work. The first
The second bit of funkiness is that we have to have some return type,
The function itself is subtly different from the PostgreSQL version,
There's a little bit of funkiness required to make this work. The first piece is that we need to modify the function registration to provide access to the connection for our internal functions. This isn't *strictly* necessary, we could `transmute` the `&self` to be `&'static self`, since the function will never be called after `self` is dropped. That felt like it would add some unneccessary unsafety though. The second bit of funkiness is that we have to have *some* return type, so we can't just return `()`. I think I want to fix this in the future by providing a `Null` SQL type, which provides `ToSql` and `FromSql` impls only for `()`. This requires adding a variant to `SqliteType` and `MysqlType` though, so it will have to be done in 2.0. The function itself is subtly different from the PostgreSQL version, since it runs even if no values were actually changed (we can't do something like `NEW IS DISTINCT FROM OLD` here). This also cannot be run on tables without ROWIDs. I think we should probably declare this as a SQL function in code somewhere for documentation purposes, but I want to wait until our docs are building again to follow up with that.