Lowell-based Sage Bank has named a veteran local banker who had been serving as its financial consultant as its new top executive, the latest in a series of leadership changes at an institution that has come under regulatory scrutiny multiple times in recent years. Diane Walker, who will also serve on Sage's board of directors, starts as president of the bank on Monday. Board members Ken Bishop and Thomas Zarrella had been acting as interim co-CEOs after former CEO Ted Pratt left the bank at the...