Allow auto scaling without making cluster vulnerable

Today there is (according to answer to a previous question) no way for a storage enabled cluster member to signal that it intends to leave the cluster and stigger re-balancing of its partitions in an safe and ordered way.

As a consequence it is a risky operation to "scale in" a coherence cluster (with the standard backup count of one we are vulnerable to data loss if a storage enabled node would fail while a node has terminated due to scale in). Having to use backup count of two is a possible work-around but results in a significant increase in memory use and reduced performance of updates...

A way to address this would be to provide a way to programatically indicate that a storage enabled node would like to leave and that this would initiate an orderly re-balancing of the partitions WHILE THE NODE IS STILL AVAILABLE to avoid data loss in the case of another node failing during the re-balancing.

Once this re-balancing is completed the node would be able to terminate and the scale-in operation completing.

Having this kind of functionality would be a big step taking coherence into the "cloud era" allowing cluters to not only scale out but also scale in to track needed capacity.

The issue names no files, tests, or entry points. Start by locating the cluster membership and partition rebalancing areas, then determine how a storage-enabled member could signal an orderly departure; done means rebalancing completes while the member remains available before scale-in termination.