Speculation and street talk are part of what goes on “inside the Beltway” in Washington, and today’s rumor has it that Lt. Gen. John R. Allen will take over as commander in Afghanistan whenever Army Gen. David H. Petraeus, who has been in charge there since last summer, relinquishes command. The Marine Corps could not confirm the report by former Washington Post correspondent Tom Ricks, who said in his blog “Best Defense” that he had heard of the Allen choice “around town.” If it became a reality, Allen would be the first Marine Corps general at the helm of U.S.…