Delete all loaded completedLog during rebooting

Now, Tachyon system has rotated editlog into **completed/** folder which exceeds configured size limitation(i.e., 1MB).   When the master rebooting, it loads those completed logs to rebuild the image. But after it all, those loaded completedlog files under **completed/** folder are not deleted. This will influence next round of rebooting consequently. 

This PR fixes this issue, and also adds one corresponding unit test.