OPENDJSDK-16: refactoring work required for affinity based load-balancer

Merged LoadBalancer and LoadBalancingAlgorithm hierarchies into a single
class hierarchy in order to make it easier to implement request load-
balancers:

* LoadBalancer - abstract class providing common functionality for
connection and request load-balancers
* ConnectionLoadBalancer - abstract class for connection load-balancers
(round-robin and fail-over)
* ConnectionLoadBalancer - abstract class for connection load-balancers
(round-robin and fail-over)
* Connections - moved common load-balancing options to this class.