Default value for TxFinality Depth

[manoranjith]

@matthiasgeihs @ggwpez Do you think we could add a DefaultTxFinalityDepth constant to the ethereum/channel package ?

[matthiasgeihs]

We could do that. Which value would you give it?

[manoranjith]

Since, we don't yet run our node on the mainnet, I think the same value 3 which we use in tests should work.