lttng: make dead tasks return the state to null instead of blocked.

lttng: make dead tasks return the state to null instead of blocked.

This will make the control flow view show a task finish instead of go
on and on and on. After this, PIDs can be more clearly re-used.

This patch updates the test cases as the behaviour of the state provider
changes.

Change-Id: Ib9fd24a868734b94cca9063c6753d88aa1185eb0
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/54415
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>