agent: ensure succeeded and discovered pairs fail the same time

This patch ensures that related succeeded-discovered pairs change to state failed simultaneously, to avoid leaving dangling pointers if one is freeed while the other is still in the conncheck list. Such transition is very rare, and only occurs in regular nomination mode, when the network conditions change between the time the pair is initially discovered and the time it is rechecked with the use-candidate flag.

Fixes: #141 (closed), #127 (closed)