“ Each employee of the United States Government or of a District of Columbia public employer furloughed as a result of a covered lapse in appropriations shall be paid for the period of the lapse in appropriations, and each excepted employee who is required to perform work during a covered lapse in appropriations shall be paid for such work, at the employee's standard rate of pay, at the earliest date possible after the lapse in appropriations ends, regardless of scheduled pay dates. ”