A few optimizations to FsNamesystem#RecentInvalidateSets

This jira proposes a few optimization to FsNamesystem#RecentInvalidateSets:
1. when removing all replicas of a block, it does not traverse all nodes in the map. Instead it traverse only the nodes that the block is located.
2. When dispatching blocks to datanodes in ReplicationMonitor. It randomly chooses a predefined number of datanodes and dispatches blocks to those datanodes. This strategy provides fairness to all datanodes. The current strategy always starts from the first datanode.