Adds supertx experiment adopted from playground

Reimplements the supertx experiment from the playground branch.
Makes it work with other experiments.

Results:
With --enable-superttx
derflr: +0.958

With --enable-supertx --enable-ext-tx
derflr: +2.25%

With --enable-supertx --enable-ext-tx --enable-filterintra
derflr: +2.73%

Change-Id: I5012418ef2556bf2758146d90c4e2fb8a14610c7