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 only assigned once in the constructor, right?