qt Dispatching Request Handler

Instead of 'qt' being handled by the SolrDispatchFilter (a Servlet Filter), it would be better implemented as a request handler, with a suggested name of DispatchingRequestHandler. This is better because:

• it keeps the servlet filter more focused / simplified (albeit just a little)
• it simplifies solrconfig.xml by removing/deprecating handleSelect="true". 'qt' is less magic, it works more explicitly.
• if you don't want to use 'qt' dispatch, simply don't use DispatchingRequestHandler
• DispatchingRequestHandler would get used by EmbeddedSolrServer but SolrDispatchFilter is not.

Credit: Hoss's idea, Erik coded a first draft