BARCELONA — Lewis Hamilton had a massive smile etched on his face. The seven-time world champion looked down from the top of the podium on Sunday as a melody of voices shouted the Italian national anthem. At one point, he wiped his eye with his towel. This was a moment that had been a long time coming. Mercedes has dominated this season, sweeping every pole position and winning all six races, five of which by the team’s Italian 19-year-old driver.