Slight redesign of packet: The virtual target client_id isn't sent any longer.
On continuing the publishing beyond the initial interval, the packet is now properly recalculated.
Removed code that was working around a missing assoc.
Shortened some internal constant names.

assoc.*:
- added Assoc_get_two_closest_entries() to search for the best good/bad node relative to a given id

rendezvous.*:
- send again: slightly randomized
- storing up to 4 matching entries for own published request
- defined the length of each piece of a packet instead of always using HASHLEN/2
- storing text&nospam_chksum, required to recalculate a packet into the next interval
- target virtual node is now calculated from the nodes already seen
- sending getnode() requests after each publishing to improve on the mark