add load balancer for read
add load balancer for read

when first read a block, select a
chunkserver randomly, if fails, try to
read from other chunkservers

linyvxiang/dfs,baidu/bfs,yvxiang/coverage_for_bfs,lylei/dfs,lylei/dfs,yvxiang/dfs,bluebore/bfs,baidu/bfs,bluebore/dfs,myawan/bfs-1,myawan/bfs-1,linyvxiang/dfs,linyvxiang/dfs,bluebore/bfs,lylei/dfs,bluebore/bfs,myawan/bfs-1,yvxiang/dfs,bluebore/dfs,00k/ToyDFS,yvxiang/coverage_for_bfs,yvxiang/dfs,baidu/bfs,imotai/bfs,imotai/bfs,yvxiang/dfs,baidu/bfs,00k/ToyDFS,yvxiang/coverage_for_bfs,myawan/bfs-1,lylei/dfs,yvxiang/coverage_for_bfs,imotai/bfs,bluebore/dfs