Revision 578507

SOLR-215 -- extracting multicore config stuff from SOLR-350.  This does three key things (that will make SOLR-350 easier to maintain)
1. extract SolrCore from SolrRequestParser -- this lets it be shared across cores
2. go back to the 1.2 interface for XmlUpdateRequestHandler
  public void doLegacyUpdate(Reader input, Writer output)
3. add a constructor to Config that lets you set the instanceDir explicitly (otherwise it forced static and needs to be specified as a property)