Johnson Memorial Corp. laid off 55 staff members, reduced the hours of 49 others and will file for bankruptcy protection as it prepares to merge with Eastern Connecticut Health Network. The corporation, which runs the ailing Johnson Memorial Hospital, agreed to sell the hospital's assets to ECHN in a $65 million deal subject to approval by state officials. ECHN operates Manchester Memorial Hospital and Rockville General Hospital and would maintain the 96-bed Johnson Memorial as a separate entity.