SERVER-13635 Improvements to RocksDB storage engine

Changes:
        - RocksCollectionCatalogEntry owner now RocksEngine
        - All metadata now resides in a single column family
        - Fixed a bug with rocks sorted data reverse iterators
          hitting an EOF when seeking past the last entry in
          an index

Closes #749

Signed-off-by: Benety Goh <benety@mongodb.com>