In what would be another in a recent string of bright spots for downtown Boston's beleaguered office market, JPMorgan Chase & Co. is close to leasing around 250,000 square feet of office space at the newly built tower above South Station, the Globe has learned. The financial services behemoth will be consolidating somelocal offices, including its wealth management office at 50 Rowes Wharf, into the 51-story skyscraper, according to four real estate industry sources speaking on condition of anonymity.