Add performance specs for Akka.Cluster.Sharding

[Horusiath]

We need a some numbers and metrics about Akka.Cluster.Sharding if we're going to go from prerelease to live with that. Ideally we should check it for both modes:

• Akka.Persistence based shards (mandatory) - this will be hard, as this value is highly dependent on journal implementation.
• Akka.DistributedData based shards (once Add DData shards to Akka.Cluster.Sharding #3081 is ready).

Setting akka.cluster.sharding.remember-entities on/off will also heavily affect results in both cases. Some of the tests cases we should check:

• How fast can we create new entities inside a single shard?
• How fast can we create new entities over spanned shards (not living on the same machine)?
• How fast can we send message to an entity living in a shard within the same node?
• How fast can we send message to an entity living in a shard on a different node of known location? (first call to unknown shard is using coordinator, then it's location is cached, so that subsequent ones make use of known shard location)
• How fast can we send message to an entity living in a shard on a different node of unknown location (first time hit)?
• How much time does it take to rebalance shard with remember-entities=off? (we can use custom ShardAllocationStrategy for sake of the tests)
• How much time does it take to rebalance shard with remember-entities=on? In this case number of entities stored within shard also matters.