timing: Cancel previous flame graph build job

timing: Cancel previous flame graph build job

Fixes bug 512195

The flame graph view was previously waiting for the previous job to
finish, waiting to acquire the lock, often in the UI thread. Now the
previous job is cancelled before acquiring the view's lock.

Change-Id: I36562822628e93560eefd26fc89cbf02d9469557
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/91086
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>