Implement PaddedVector

Implement PaddedVector

Implements a vector-like container that maintains padding such that
SIMD memory operations will always use initialized memory. All use of
the padded form data takes place via an ArrayRef, so that this is
explicit at the point of creation of the view. The PaddedArrayRef view
creates some safety through strong typing, also.

Increased the timeout for TSAN tests, which now take longer. Since
there are now two special case configurations for testing where we
permit a longer timeout, simplified the way we implement the increase
for the OpenCL case.

Fixes #2642

Change-Id: Ibcafe161689f4a02f7aa6fd0410edec47300264e