Recognize when the local peer is the one "missing"

[jimkberry]

P2pNet should be able to recognize when "missing" peers actually mean that the local peer has gone offline.

Currently it will never report the local peer as being "missing"

[jimkberry]

There will likely be some heuristics.
Can it even be done for non-tracking channels?

[jimkberry]

Couple of thoughts:

If every peer is missing from their shortest-timeout channel:
    then we are maybe missing
    if that is more than 1 peer:
        then we are probably missing
else:
    we are not missing

• OR -

If net-wide "last heard from anyone" as long or longer than the shortest `missingMs`:
  we are missing

Not super-sure about either of these