CDAP-633 RunId of Currently Running Program 

Based on the discussion at
https://issues.cask.co/browse/CDAP-633

history endpoint is moved to /runs endpoint, with default call to runs will return all currently active and past runs of a program.

while /runs?status={completed/failed/active} can be used to filter the returned records.

This PR contains CLI command change and UI changes as well.
Why nullable? Shouldn't be.