Allow keys other than _id for primary key in MongoItemWriter

[blake-bauman]

When moving data between different data store types, such as between MongoDB and a SQL database, not all systems have a "_id" as their pimary key. Most often it has a different name or can be multiple keys/columns used as a composite key. This change allows the app to specify an alternate key or a set of keys which, together, can be used as the primary key for a MongoDB insert/update/upsert.