Doctors across the US are stretching to keep their practices afloat as a debilitating cyberattack on a once little-known company at the center of the health-care system continues to cause havoc.The Feb. 21 attack against Change Healthcare, a subsidiary of the largest US health insurer by market value, UnitedHealth Group Inc., has seized the health-care system for three weeks and counting, halting the normal flow of billions of dollars in payments between doctors, hospitals, pharmacies and insure...