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
id_hash() was not at all working as expected for very small bucket size (when (size / 4) was zero). Simplified to be trivially correct.
Also added a used flag on adding an entry, which is set by callers if they have that association in active use. Those get priority over unused entries on collision.
Fleshed out test to be at least elementary useful.
Each group chat now uses an own, small assoc (80 entries).
