Modified the documentation.
Change it from an interface to an abstract class in the document.
Rename a method to isEnabled.
Add a class to repsent the enabled engine list.
use ContentTypeInputStream interface (and ByteArray implementation) to specify the desired content-type of the InputStream returned by MN.query().
Remove the code to throw an exception if the subjects is null in the query method.
changed the xpath to get solr_spec_version.
Add the getIndexFields and getValidIndexFields methods.
Add getSolrSpecVersion method for the HttpServer and the controller.
Imeplement the getSolrVersion, getSchemaField and getValidSchemaFields for the EmbeddedSolrServer.
move SolrQueryResponseTransformer and SolrQueryReponseWriterFactory to the query package.
Remove the query(String query) methods from SolrQueryService.java.
Remove the depency on the d1_cn_index_processor.
Use one query method in the SolrQueryServiceController.
Move the solr query part here since metacat-index needs to query the solr server too.