Reduce number of holepunching attempts when no direct connection exists

[flub]

Currently iroh keeps trying to holepunch every 5 seconds if there is no direct connection. Until eternity. That's a lot of sad network traffic (that might even get iroh flagged) for something which we know probably won't work.

Probably a better approach is to do some kind of exponential backoff algorithm on holepunching attempts, combined with resetting the backoff as soon as a network change is detected for either peer.