UPnP port mapping should be regularly attempted, not only at startup

[attila-lendvai]

Summary

my upnp service may become unresponsive (miniupnp in openwrt). when i notice i restart it, but seems like i also need to restart all my bee nodes, because it seems to only attempt (?) setting up the upnp mapping at startup.

i waited about 10 mintes only before restarting the bee nodes.

Motivation

make bee nodes' external reachability more reliable.

Implementation

i'm no expert neither of golang nor upnp.

maybe it's as simple as moving the setup code from the initialization into a periodic thread?

Drawbacks

i'm not sure whether the upnp API is smart enough for such a use-case (i.e. gracefully handling when the same port map is requested periodically).