async getBalance() {
    let balance = (await this._redis.get('balance_for_' + this._userid)) * 1;
    if (!balance) {
      balance = await this.getCalculatedBalance();
      await this.saveBalance(balance);
    }
    return balance;
  }
LndHub no longer relies on redis balance as source of truth, this is
more a cache now. See `this.getCalculatedBalance()` to get correct balance.

@returns {Promise<number>} Balance available to spend