Auto probing of local channels

[pm47]

This is a simple mechanism to test our direct peers by sending fake
small payments to them. A probe is considered successful if the peer
replies with an UnknownPaymentHash error.

[akumaigorodski]

While at it, what do you think about ACINQ/eclair-mobile#94?
This behavior may be specified in config, it has proven to be a good bug hunting tool in my case.

[pm47]

But the point here is to close zombie channels right?

[akumaigorodski]

To spot them, yes. What happens next may differ depending on whether this happens on server side or mobile app. In a letter case user may be interested to keep the channel open and let peer fix the bug.

[pm47]

I'm not sure this probing should be enabled on eclair-wallet, it makes more sense for always-on nodes

[pm47]

I think this feature could be merged, but disabled by default because:

• nodes aren't very reliable on current mainnet and this would probably cause too many channels to get closed for our users
• there is currently no way to tell destination node it is being probed, so that could cause unnecessary support work