v5.8.5

This is a small update that introduces a few features and fixes some flaws:

• a new method so_drop_all_subscriptions_and_filters for agents;
• a new agent tuning option skip_demands_on_dereg (#96);
• empty_notificator for mchains;
• the mchain_receive_result_t is now structured binging friendly (thanks to @ilpropheta);
• a new field in so_5::stats::activity_stats_t structure.

More detailed descriptions of new features can be found on the corresponding Wiki page.