Link GPU force producer and consumer tasks

Link GPU force producer and consumer tasks

The GPU event synchronizer that indicates that forces are ready
for a consumption is now passed to the GPU update-constraints.
The update-constraints enqueue a wait on the event in the update
stream before performing numerical integration and constraining.
Note that the event is conditionally returned by the
StatePropagatorDataGpu and indicates that either the reduction of
forces on the GPU or the H2D copy is done, depending on offload
scenario on a current timestep.

Refs. #2816, #2888, #3126.

Change-Id: Ic12b0c55b75ec5f0c31ce500a2760fb4d5cf3b91