MEDIUM: cpu-topo: switch to the "performance" cpu-policy by default

MEDIUM: cpu-topo: switch to the "performance" cpu-policy by default

As mentioned during the NUMA series development, the goal is to use
all available cores in the most efficient way by default, which
normally corresponds to "cpu-policy performance". The previous default
choice of "cpu-policy first-usable-node" was only meant to stay 100%
identical to before cpu-policy.

So let's switch the default cpu-policy to "performance" right now.
The doc was updated to reflect this.