onFundingLoanClose ({ symbol, cbGID }, cb) {
    this._registerListener('flc', { 1: symbol }, FundingLoan, cbGID, cb)
  }
Register a handler to be called when a funding loan is closed

@param {object} opts - options
@param {string} [opts.symbol] - symbol to match
@param {string} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-auth-loans