We don't have vectors re-allocation happening multiple times from inside
a loop anymore, so we can safely switch to a memory guarded allocator for
vectors and keep track on the memory usage at various stages of rendering.

Additionally, when building from inside Blender repository, Cycles will
use Blender's guarded allocator, so actual memory usage will be displayed
in the Space Info header.

There are couple of tricky aspects of the patch:

• TaskScheduler::exit() now explicitly frees memory used by threads. This is needed because threads is a static member which destructor isn't getting called on Blender's exit which caused memory leak print to happen.
  
  This shouldn't give any measurable speed issues, reallocation of that vector is only one of fewzillion other allocations happening during synchronization.

• Use regular guarded malloc (not aligned one). No idea why it was made to be aligned in the first place. Perhaps some corner case tests or so. Vector was never expected to be aligned anyway. Let's see if we'll have actual bugs with this.