PATCH: XSLTServletFilter

Patch that implements server-side XSLT transforms of query results.

The filter is activated by using select/html instead of select/ to run queries, and optionally adding a transform parameter to specify the XSLT transform to use, for example:

http://localhost:8983/solr/select/html?q=usage&transform=my.xsl

In which case my.xsl should be found in solr/conf/xslt/ with the example configuration. The default transform (solr/conf/xslt/query-to-html.xsl) outputs a simplistic HTML format.

Performance is suboptimal, as the filter reparses the XML output generated by Solr. Modifying the XMLWriter to output to a ContentHandler would be more efficient, but I didn't have time to go that far.

The TransformerProvider trivially caches the last Transformer used, could be improved using an LRU cache of several transformers, I haven't checked if Solr's infrastructure contains such an animal already.

The patch is all new files, except for adding this in web.xml before the first <servlet>:

<filter>
<filter-name>xslt</filter-name>
<filter-class>org.apache.solr.xslt.XSLTServletFilter</filter-class>
</filter>

<!-- apply the XSLT filter when select/html is used to make queries -->
<filter-mapping>
<filter-name>xslt</filter-name>
<url-pattern>/select/html/*</url-pattern>
</filter-mapping>

I've left the client-side XSLT stuff (stylesheet parameter) as is for the moment.