[contracts] Sanitize ethService for 3rd party use

[LayneHaber]

Describe the bug ethService is not usable by third parties (i.e. watchtowers), and should be updated to be friendlier to users outside of the channel.

For instance in sendDepositTx:

    if (channelState.alice !== sender && channelState.bob !== sender) {
      return Result.fail(new ChainError(ChainError.reasons.SenderNotInChannel));
    }