vp9-svc: Allow usage of second (long term) temporal reference.

vp9-svc: Allow usage of second (long term) temporal reference.

Allow for second temporal reference for top spatial layer in SVC,
when inter-layer prediction is disabled on INTER frames.
The second temporal reference is labelled as the golden reference
and the update/refresh of this reference buffer is only on base
temporal layer superframes. For now the period of refresh is
fixed at every 20 TL0 superframes.

Average gain is ~4% on RTC set, several clips up
by ~8-12%. Speed loss is about ~2% on mac.

Feature is disabled as default for now.

Change-Id: I2e5db5052c62dbe958a3b14be97d043823b7a529