When 1st time launch, read from the TAIL instead of HEAD

1. (Map<Object,Object>)JSONValue.parse(new String(b, "UTF-8")); will never throws any Exception, because it has already catched any exception threw, and returned null. 
So remove the comments that trying to say parse fail.

2. Read operation probably throws Runtime exception, and under this condition, I think we should read from the HEAD to avoid duplicate commit.

3. Also, there's one more possibility that there is some data under the commit path but not valid. So jsonTopologyId or jsonOffset is null. For this, I suppose it would be the error from last time writing or manmade by purpose? Anyway, We couldn't let last time error impact current operation. So pick the HEAD too.