Bug 432049: JobGroups API and initial implementation.

Change-Id: Id1162cd9f084128db075780a15fb537e761722b2
Signed-off-by: Thirumala Reddy Mutchukota <thirumala@google.com>
Signed-off-by: Terry Parker <tparker@google.com>
Moved all job group lock access into the job group class
Minor cleanup, making unchanged fields final
Minor cleanup