Billy Galvin has been at the Statehouse longer than Josh Zakim has been alive. Not that there is anything wrong with that. Except for the possibility that Galvin, the veteran secretary of state, may for the first time in his long political career face serious Democrat primary opposition. And the question is: Does Galvin face a bridge too far? The challenger is Josh Zakim, 33, a Boston city councilor, who is seeking to oust Galvin. If the name is familiar it is because the Zakim Bridge is named after his father. Ordinarily, a bridgeless Zakim would be no big deal....