[web] write css_class to attrs on rendering and not on button creation Because css_class attribute may be overriden by bootstrap after button creation and this modification has to be taken into account. closes #5457548

[statsd_logger] handle ipv6 addresses

implement a simple statsd logger (closes #5488711) and add a few timing probes on: - etwist.server.CubicWebRootResource.render_request() - server.querier.QuerierHelper.execute() - server.sources.native.NativeSQLSource: - sqlexec() - compile_rql() - authenticate() - doexec() - doexecmany() as well as a pair of counters on server.querier.QuerierHelper cache (hit/miss).