When deleting a large directory with millions of files, namenode holding FSNamesystem lock will make it unresponsive for other request. In this scenario
HDFS-173 added a mechanism to delete blocks in smaller chunks holding the locks. With new read/write lock changes, the mechanism from HDFS-173 is lost. Need to resurrect the mechanism back. Also a good unit test/update to existing unit test is needed to catch future errors with this functionality.