addto_lists(): store the IP/Port that was used to *send*.

Avoids a DOS of sending a copy of a valid response with an invalid IP.
Behind #define LOGGING: Correct usage of now() to replacement unix_time().
Reduce duplicate data. Add a mostly empty skeleton test.
Unify most of handle_sendnodes[_ipv6]() into a common handle_sendnodes_core(), leave only a minimal trail.
Merge pull request #640 from FullName/logging_fixes

Behind #define LOGGING: Correct usage of now() to replacement unix_time().
Merge pull request #641 from FullName/autotests_reduce

Reduce duplicate data. Add a mostly empty skeleton test.
Merge pull request #642 from FullName/dht_handle_sendnodes_unify

Unify most of handle_sendnodes[_ipv6]() into a common core.
Merge branch 'add2lists_forward' of https://github.com/FullName/ProjectTox-Core into FullName-add2lists_forward
Code cleanups.