A congressional panel will grill two former Massachusetts U.S. Attorneys next month about the government's ties to mob informants while the taxpayers' bill for related civil suits is soaring. The House Committee on Government Reform announced yesterday it will hold a Dec. 5 hearing in Boston to question former top prosecutors Paul Markham and Jeremiah T. O'Sullivan. Markham led the U.S. Attorney's Office during mob prosecutions in the 1960s while O'Sullivan was U.S. Attorney when the FBI used informants James ``Whitey'' Bulger and Stephen ``The Rifleman'' Flemmi during the 1980s. ``These events are extraordinarily important and Congress must be thorough...