Make RT mode encoding for VP8 be stable.

In mode --rt, and with --cpu-used set to >= 0, the vp8 encoder will
create different files depending on the machine load. Setting --cpu-used
to a negative number alleviates this.

This CL also contains a tool for verifying that a configuration is
stable, as well as a fix to the hashname verification that checks that
the configuration is unchanged by the ConfigurationFixups call.
If there was no encoding stored, shouldn't you return false by definition since "Verifies whether any encodings have changed compared to what's in the database."