tmf: Early-exit event search methods if monitor is cancelled

tmf: Early-exit event search methods if monitor is cancelled

Prevents from sending a request at all if the job is already
cancelled when the methods get called.

Fixes a spurious test failure, but also covers for the case
where the job could be cancelled very quickly.

Change-Id: I7dd17fd32663ad6985ad4572541e896e9173dc7d
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/77106
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI