Juwang http perf request REEF-29

After client started job driver, clients would like to know the progress of the job execution and how much time is spent at each stage. This PR is to support such http requests. The data is obtained by parsing the scope logs.

It also allows the user to access log files through http request.
 
This is to resolve issue REEF-29 https://issues.apache.org/jira/i#browse/REEF-29?filter=-1

It is true for now. As log scope can be applied anywhere in the class. I usually inject it in constructor and keep it as an instance variable so that it can used later in other methods as well.