vp8: Suppress denoising with respect to old reference frames.

If the GOLDEN or ALTREF frame was last updated > x frames in the past,
don't use them for denoising (only consider LAST). Using an old reference
frame for denoising, e.g., if it is a long-term reference or the last key frame,
can cause some visible artifacts, in particular in the aggressive denoising mode.

Change-Id: I239c9fbb092c36cba7e95328f1fa67a58d6a7ed9