#761 Fix removal and restoration of primary permission group

- Improve how a player is being switched between permission groups (add new group before removing old one)
- Remove group handling logic from LimboCache: AuthGroupHandler is now solely responsible for changing the player's permission group

src/test/java/fr/xephi/authme/service/CommonServiceTest.java

@@ -134,13 +134,11 @@ public class CommonServiceTest {
         // given
         Player player = mock(Player.class);
         AuthGroupType type = AuthGroupType.LOGGED_IN;
-        given(authGroupHandler.setGroup(player, type)).willReturn(true);
 
         // when
-        boolean result = commonService.setGroup(player, type);
+        commonService.setGroup(player, type);
 
         // then
         verify(authGroupHandler).setGroup(player, type);
-        assertThat(result, equalTo(true));
     }
 }