configurable loadBalancer for ClientConfig

- the default is still HeapBalancer
- you create your own by extending LoadBalancerFactory

Conflicts:
	finagle-core/src/main/scala/com/twitter/finagle/builder/ClientBuilder.scala
	finagle-core/src/main/scala/com/twitter/finagle/client/DefaultClient.scala
	finagle-core/src/main/scala/com/twitter/finagle/loadbalancer/HeapBalancer.scala
bump version to 6.3.0_fs-a