Pass list of EventSynchronizers to GPU reduction

Pass list of EventSynchronizers to GPU reduction

Instead of passing events one-by-one or doing implicit synchronization
hidden in a force tasks' internals, a list of synchronizer objects that
signal the availability of forces from the source tasks is assembled and
passed to the GPU buffer ops/reduction.

The new dependency added here is the H2D transfer for CPU-side force
contribution, PME-related event is kept, halo exchange dependency needs
to be added.

Change-Id: I5ca5ef919b49508790e813e0469aaeef4f6484c0