Move ConnectionIdentifier to util package.

I also replaced the use of MongoIdentifier with ConnectionIdentifier.

Then, I added RecordRules.fieldName that takes a ConnectionIdentifier as an argument. I had to add `def connectionIdentifier` to MetaRecord for this to work.

Replaces #1550 

One question: DefaultConnectionIdentifier.jndiName was defined as a `var`. Does anyone know if that was intentional? I changed it to `val` here.

After I did this I realized we don't necessarily need to deprecate db.ConnectionIdentifier. We could just leave the type alias the same way mapper is.