Make a list of default electrum node and fallback to one of them if the main one is unstable

[devbordecraft]

Kudos to @binarybaron

Great, thanks for taking the initiative to fix this. I think it might be useful to have a list of Electrum nodes and fall back to another server if the primary one fails.

Originally posted by @binarybaron in https://github.com/comit-network/xmr-btc-swap/issues/866#issuecomment-991933677