fix OwnerBalance calculation

[NiloCK]

complete this todo in StateChannelWallet

  async getOwnerBalance(): Promise<number> {
    const walletBalance = await this.getBalance();
    // todo: subtract also the pending HTLCs that target the interface
    return walletBalance - (await this.getIntermediaryBalance());
  }

[geoknee]

@NiloCK is the idea that "in flight" htlcs (signed into the ledger channel state, but not yet unlocked) should be taken into account for the balances?

If so, I think for the demo it may not be worth doing since htlcs get unlocked so quickly?