Add option to make balancer prefer to get cold blocks

We met two issues when using balancer.

1. Moving hot files may cause failing of dfsclient reading.
2. Some blocks of temporary files are moved and they are deleted soon.

Add a config key  " dfs.namenode.hot.block.time.interval",  the balancer prefer to get the blocks which are belong to the cold files created before this time period.

Also add a option "-hotBlockTimeInterval" to the command line of balancer for inputting.