TMF: bug 440998: Set active task to null in TmfRequestExecutor when completed

TMF: bug 440998: Set active task to null in TmfRequestExecutor when completed

This avoids keeping references to the request, which itself may contain
references to rather big objects.

Change-Id: Ib82fbea5cdc6b73a0a76d51bc2e960d0d223927a
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/30903
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>