Improve speed and stability of LAN discovery test

[iphydf]

In particular, speed up the lan_discovery_test. It often takes 1.5 minutes, and in the appveyor (windows/msvc) build it often times out completely.

[zoff99]

last time i looked i think i found that LAN discovery happens per friend and not globally.
and also it happens after a friend is already online.
need to confirm if i made a mistake. but if so those would be nice to change those.

[iphydf]

Fixed in #2126.