gossip: allow 'retry' in select_data() callback
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.
seyyed/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,tectronics/scalaris