Make the load balancer run with a gentle hand

We need 'smoothing' of balancer region move.... Yesterday we brought a regionserver back online into a smallish cluster that was under load and the balance run unloaded a bunch of regions all in the one go which put a dent in the throughput when a bunch of regions went offline at the one time. It'd be sweet if the balancer ran at a context appropriate 'rate'; when under load, it should move regions 'gently' rather than all as a big bang (the decommission script will move a region at a time, verifying it deployed in its new location before moving another... this can take ages to complete but its proven minimally disruptive to loadings)