Admission control tests: Increase thread join timeout and remove unnecessary locking

In some rare cases on overloaded machines, the thread join timeout of 10 seconds isn't
long enough. Also, taking the lock at that time isn't necessary because the main thread
will not attempt to cancel a thread unless it is already in the list of running threads.
Threads are added to that list only after they submit their query.

Change-Id: I23a67d726bc25221f0e9331ca1a3e9f5363f821d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1744
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins