gossip2: fix bug in callback module startup
gossip_load: fix bug in calc_stddev() and calc_size_kr()
gossip: make callback modules (gossip tasks) instantiable

Includes global starting and stopping of callback modules (gossip tasks).
gossip: add requestable histograms to gossip_load
gossip: refactor instantiability
gossip: uncomment gossip2 in sup_dht_node and dht_node_join

Signed-off-by: Jens V. Fischer <jensvfischer@gmail.com>
gossip: activate gossip2 instead of gossip in webhelpers

Signed-off-by: Jens V. Fischer <jensvfischer@gmail.com>