Improve Plugin servlet filter functionality

Openfire plugins can contain Servlets. Servlet Filters are also available, but in a limited form. Openfire ships with a definition that's aptly named 'SimpleFilter' - it does not quite provide all features that regular Servlet Filters do.

The limited nature of SimpleFilter prevents third-party Servlet Filters to be used by Openfire plugins, as SimpleFilter does not extend javax.servlet.Filter.

Plugins should be able to use third party Filters.