Add uniqueKey parameter to Solr Cell, allowing client to work without knowing the actual field name

This comes from an LCF need of indexing content generically into Solr, where the LCF side does not know the field name of the schema.xml uniqueKey specified field. You can send in literals to Solr Cell, such as literal.id=my_unique_key, but this relies on the client knowing that "id" is the field name.

Let's add a parameter to Solr Cell, such as uniqueKey=my_unique_key, and underneath Solr Cell will map that to the proper field.