async saveBalance(balance) {
    const key = 'balance_for_' + this._userid;
    await this._redis.set(key, balance);
    await this._redis.expire(key, 1800);
  }
LndHub no longer relies on redis balance as source of truth, this is
more a cache now. See `this.getCalculatedBalance()` to get correct balance.

@param balance
@returns {Promise<void>}