Provide reason for needing rebalance

The "balanced" field returned by /pools/default is key as it used by external entities to determine if the cluster needs rebalancing. When the "balanced" is "false" it is not always apparent the reason. The higher level checks: a service needs rebalance or buckets need rebalancing. But each of those has multiple checks...the active nodes for a service doesn't match the expected nodes for the service, a topology aware service indicates it needs a rebalance, the number of replicas for bucket changed, etc.

It would be useful if /pools/default would return the reasons (or maybe just the first reason if other checks are short-circuited when the first is encountered). e.g.

 "balanced": false,

 "reason": "FTS needs rebalance",

We should also log this information at the start of each rebalance in the debug logs.