async unlockFunds(pay_req) {
    let payments = await this.getLockedPayments();
    let saveBack = [];
    for (let paym of payments) {
      if (paym.pay_req !== pay_req) {
        saveBack.push(paym);
      }
    }

    await this._redis.del('locked_payments_for_' + this._userid);
    for (let doc of saveBack) {
      await this._redis.rpush('locked_payments_for_' + this._userid, JSON.stringify(doc));
    }
  }
Strips specific payreq from the list of locked payments
@param pay_req
@returns {Promise<void>}