Possibility to index document into two solr servers of different version

[mhaslauer]

In order to enable migration strategies from one Solr version to another, it would be helpful if Vind supports indexing into two Solr servers of different version at the same time. In such a case, an application could build up the index in the new Solr server in parallel to an already existing one. As soon as both Solr server contain the same amount of documents, the application could switch to the new Solr server for querying.