tmf: Use a foreground request for partial history checkpoints

tmf: Use a foreground request for partial history checkpoints

This will ensure the checkpoint request does not get coalesced with
the "real" state history building request, as this would lead to
a deadlock.

Change-Id: I69f6ab52a75a3344a60869ac2b11332581afddd3
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/20836
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Hudson CI