Resistant contract deployment

[ggwpez]

Location: [backend/eth/channel/] deploy.go

Problem: When deploying a contract we currently assume it to be final after one block inclusion (bind.WaitDeployed).
In order to archive reorg resistance, we need to confirm the TX analog to #97.

Solution: Create a way to wait for more than one block, in the best case by reusing the code from #97 but possibly with a different finality depth.