Merge common nbnxn CUDA/OpenCL GPU wait code-paths

Merge common nbnxn CUDA/OpenCL GPU wait code-paths

The entire GPU wait including timing accumulation as well as staging
data reducion of the nonbonded GPU modules has been unified by
including a single templated version of the code into the common header.
Code has only been moved and changed in minor ways when necessary (e.g.
for the rvec reduction).

Change-Id: Ic9c9690be58a78f92ca99d2af30068e19c19cc6c