async lockFunds(pay_req, decodedInvoice) {
    let doc = {
      pay_req,
      amount: +decodedInvoice.num_satoshis,
      timestamp: Math.floor(+new Date() / 1000),
    };

    return this._redis.rpush('locked_payments_for_' + this._userid, JSON.stringify(doc));
  }
Adds invoice to a list of user's locked payments.
Used to calculate balance till the lock is lifted (payment is in
determined state - succeded or failed).

@param {String} pay_req
@param {Object} decodedInvoice
@returns {Promise<void>}