Make hard coded parameters used by balancer and other tools configurable

During investigation of how to speed up balancer, at least to the level specified by dfs.datanode.balance.bandwidthPerSec, we found that parameters such as MAX_BLOCKS_SIZE_TO_FETCH and SOURCE_BLOCKS_MIN_SIZE are hard coded. These parameters are related to block size and other configurable parameters used by balancer. So least we should make it configurable. In the longer term, it might be interesting to understand if we simplify all these related configurations.