Improve dmap get load all blockkeys

 Enable/Disable Offset preloading
    
        * Refactored DMap class to enable/disable offset preloading during
          map load. If enabled, DMap loads all Key-offsets information into
          memeory. Else, DMap maintains Mapped byte buffer of block trailers
          allowing to do a linear search.
    
        * Improved Benchmark class to run with both offset enabled/disabled
          for various block sizes and keys.