Allowing import / update of a specific document using the data import handler

There is the need that changes or new documents need to be added immediately to the Solr Index. This could easily done via the update-handler - however, when using the DataImportHandler it shouldn't be necessary to specify the data extraction for the the DataImportHandler and also for feeding it to into the update-handler. It should be centralized.

Having to run delta query, identifying the changes, for changes where the ID's of the updated documents are already known to the application is a rather costly (in terms of database load) way to solve this.

The attached patch allows to specify one or more query parameters for the delta-import command, named 'root-pk', which allow to specify the document(s) to be updated or added.