Squashed Changes
Changed to extends AbstractStoreOutput
Refactored property names
removed incorrect LOG statements
code review changes
Merged pull request #992
MLHR-1366: added test to retrieve latest value
MLHR-1367 #resolve

 * Added aggregate counters for AbstractFSDirectoryInputOperator
 * Implemented counters with basic counters
 * added javadoc
 * fixed NullPointerException in stats listener of AbstractThroughputFSDirectoryInputOperator
 * Cleaned up warnings in AbstractThroughputFSDirectoryInputOperator