make whitelist of params that DUP will forward (in filterParams(...)) amendable by other processor factories

DistributedUpdateProcessor is currently very conservative in what params it will copy when creating requests for other nodes, using a hardcoded internal list of only 2 param names. There are good reasons for this behavior in general, but it's really limiting – especially when you're implementing a custom RunAlways UpdateRequestProcessorFactory (or an update processor that you expect to be configured after DUP).

This situation has come up for me in SOLR-445, so I'd like to add support for a new method that Factories can use to communicate with DUP regarding the params they expect it to forward.