tmf: Fix possible concurrency issue with event request IDs

tmf: Fix possible concurrency issue with event request IDs

Protect accesses to the static field with a static lock, to make sure
that all requests really have different IDs. Now that coalesced
requests are put in a Set, the IDs matter.

Change-Id: Iccb226aa6969c8150e0c8cc506b8c68a94e902ee
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/37759
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>