Dave Grohl. Superstar road-tripper next door? Old-school recording studio preservationist? 21st century rock-and-roll diplomat? He’s all of those things in “Sonic Highways,” a new HBO rockumentary series that follows the 45-year-old rock mensch and his Foo Fighters as they traverse America’s asphalt arteries, recording their new album. Eight songs, eight recording studios, eight different cities.