[TACHYON-162][TACHYON-164]We could directly load edit log

https://tachyon.atlassian.net/browse/TACHYON-162

We could load edit log when checking whether the *.editlog does exist or not, instead of storing their path in array and iterating it again.
Should this use File or "UnderFileSystem"? @calvinjia @WangTaoTheTonic