Feature/site map generator

Please validate the approach .
SiteMapGeneratorImpl is the service factory holding the configs for each site.
SiteMapGeneratorServlet will redirect to appropriate service using the domainName from host header.
SiteMapFilter will forward all "/sitemap.xml" requests to the servlet.
PageReplicationEventListener will listen for replication of the any page under the configured domain and issue a resourceonly flush request to flush out sitemap.xml in the web server cache