Add new ShardsAllocator that takes indices into account when allocating and balancing shards

[s1monw]

The current EvenShardCountAllocator balances shards in a global fashion across all eligible nodes in the cluster. Under certain circumstances this algorithm can overload certain nodes with multiple shards for a single index while still gaining global balance.
The shard allocation algorithm should be more flexible to take into account more information like indexes or primaries and eventually things like the size of a shard or request patterns of an index.