Add a ClientPool for distributing to multiple gearmand

[bzikarsky]

• Add a class ClientPool (implementing ClientInterface); additional methods
  • addClient($hostname, $port)
  • setDistributionStrategy($strategy)
  • getDistributionStrategy()
• Add a factory method createClientPool($strategy = null)

The client-pool should chose a client at submission based by its strategy, e.g. random, workload-hash-based or just the first one. Actions such as setConnectionOption should forwarded to all clients. The actions should be robust against single Clients disconnecting/failing and move on accordingly,