Patch from Erich Focht

This adds a hook to rebalance globally across nodes every NODE_BALANCE_RATE
iterations of the rebalancer. This allows us to easily tune on an architecture
specific basis how often we wish to rebalance - machines with higher NUMA
ratios (more expensive off-node access) will want to do this less often.
It's currently set to 100 for NUMA-Q and 10 for other machines. If the
imbalance between nodes is > 125%, we'll rebalance them. The hook for this
is added to the NUMA definition of cpus_to_balance, so again, no impact
on non-NUMA machines.