Provide a way to exclude subtree from balancing process

This patch adds -exclude <path> option to balancer tool, and, if provided,
balancer will not move blocks under this path during operation.

It it extremely usefull for HBase, which allows to rest of
HDFS without breaking HBase data locality.
Fix possible NPE in excludeBlocks refresh
Fix anover possible NPE
Speed up balancer by early exclude block