Implement 2-week network view pruning

[blico]

An amendment was recently made in the set of specifications to recommend that nodes prune stale entries from their channel graph as it's possible for channel to become completely inactive for a variety of reasons.

What this change includes:

• Within AuthenticatedGossiper's ChannelUpdate re-broadcast loop added a prune check, which prunes an edge from the graph if it is at least 2 weeks old.
• Modified the main ChannelUpdate re-broadcast loop to only rebroadcast a ChannelUpdate if it has not been updated for at least 13 days.
• Created an updateChannel function which creates a new ChannelUpdate, updates the edge Timestamp field, re-signs the channel, and writes the updated ChannelEdgePolicy to disk. This function is used by both processFeeChanUpdate and the re-broadcast loop.

[mention-bot]

@blico, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Roasbeef, @AndrewSamokhvalov and @cfromknecht to be potential reviewers.

[jimpo]

The refactor of processFeeChanUpdate into updateChannel looks good to me.

With regards to the pruning, however, I believe the idea is to prune channels that we are not a participant in, whereas this only prunes our own outgoing channels. In fact, after this change that should no longer be possible since the edge is updated before the prune interval. I think instead you want to use Graph.ForEachChannel and it probably no longer makes sense to do it at the same time as the rebroadcast .

Not sure whether the resigning channel updates every 13 days is a good idea or not, but it makes sense to me.

Notes:

• Rebroadcast interval is changed from every 30 minutes to every 13 days.
• All outgoing channels are automatically every 13 days to prevent pruning.

[Roasbeef]

Thanks for doing the initial work to implement this @blico!

I've picked this issue up, and implemented the fixes for the review items @jimpo pointed out, and also added some initial functionality. I've left your original commits in place in order to ensure you get credit for the initial work.

Check out https://git.io/vdBkH to see the of commits pushed.