=con #3880 Keep track of all shards per region in ClusterSharding

* The problem was that ShardRegion actor only kept track of one shard
  id per region actor.  Therefore the Terminated message only removes
  one of the shards from its registry when there are multiple shards
  per region.
* Added failing test and solved the problem by keeping track of all
  shards per region
* Also, rebalance must not be done before any regions have been
  registered
Merge pull request #2019 from akka/wip-3880-sharding-terminated-patriknw

=con #3880 Keep track of all shards per region in ClusterSharding