gossip: refactor trigger handling

Refactor trigger handling to allow trigger_action messages for individual modules.
Needed for queing trigger_action messages for individual modules.
gossip: allow 'retry' in select_data() callback

Allow callback modules to queue select_data calls for retrial by returning
{retry, State} to the select_data() callback.
gossip_cyclon: Fix select_data() by returning 'retry' on empty caches
gossip_cyclon: Add printing of caches in graphviz/dot compatible syntax
gossip_cyclon: Add script to create dot graphs from cyclon chaches
gossip_cyclon: implement handle_msg(rm_changed)
gossip_cyclon: comment out some debug messages
gossip_cyclon: implement handle_msg(get_ages)

Also adjust get_ages request for in admin to fit gossip_cyclon (commented out)
gossip_cyclon: implement get_subset_rand
gossip: change msg tag for messages to callback modules

Change msg tag for messages to callback modules from cb_reply to cb_msg, as it
is not only used for replies, but also for messages directly to the callback module.
gossip_cyclon: implement handle_msg(get_dht_nodes_response)
gossip_cyclon: add config values and check_config
gossip_cyclon: implement web_debug_info
gossip_cyclon: refine type specs
gossip_cyclon: change calls to get_subset_rand (commented out)
gossip_cyclon: uncomment get_subset_rand calls to gossip_cyclon
gossip_load: fix typechecking, add init_feeder
wip typechecking