Amazon has fallen so far behind schedule in creating new jobs at its Northern Virginia headquarters that its workforce at those offices shrank last year, the company confirmed, underscoring how a project that it had initially pitched as an economic jolt is instead hitting a slowdown.Following a much-hyped sweepstakes across North America several years ago, the tech giant made a deal with state and local officials to locate half of its “HQ2” in Arlington, just outside D.C.: In exchange for as muc...