Add subset filter for server list

Provides a server list filter that limits the number of the servers used by the load balancer to be a relatively fixed subset of all servers. This is useful if the server farm is large (e.g., in the hundreds) and making use of every one of them and keeping the connections in http client's connection pool is unnecessary. It also has the capability of eviction of relatively unhealthy servers by comparing the total network failures and concurrent connections.