Implement ElasticShardSpec

[gianm]

From https://groups.google.com/forum/#!topic/druid-development/kHgHTgqKFlQ,

Create an ElasticShardSpec that has two numbers per shard: partitionNum and corePartitions. The idea is that when a set of shards is first created, corePartitions = max(partitionNum) + 1. After that point, corePartitions never changes. It has the same value on every shard. The purpose of this is to make replacements atomic while still allowing for shard set expansion; brokers can treat a set as “complete” if it has all of its corePartitions. The idea is that ElasticShardSpec would actually be used for everything: both realtime ingestion and batch ingestion.