get_close_nodes(): make use of assoc to find some really close nodes

DHT.c:
- get_close_nodes(): use assoc to find nodes for the requested reference id
- handle_sendnodes*(): set a proper timestamp for "heard" nodes

assoc.*:
- expand Assoc_close_entries by flags for additional restrictions for search
- Assoc_get_close_entries(): honor the new flags
- new_Assoc() prime handling: min. prime is now 5 instead of 3, and up to 25 is handled directly
- Assoc_status(): added address family for seen/heard, print of hash is now fixed width