MetadataCleaner - fine control cleanup documentation

Documentation with reference to commit b5025d90bbf3e81701a3c3f5e22efbb2e2164eea by Mridul
Date:   Fri Oct 4 21:57:57 2013 +0530
    - Allow for finer control of cleaner
    - Address review comments, move to incubator spark
    - Also includes a change to speculation - including preventing exceptions in rare cases.