Target Explorer: Fix race conditions when canceling stepper jobs

- Avoid manipulating JobMonitor after job has finished
- Spurious ConcurrentModificationException in CancelJobsStep

Change-Id: Ia7baea03fc9f9ce093d9f26ba3e176e1405246ff
Signed-off-by: Anton Leherbauer <anton.leherbauer@windriver.com>
3 files changed