making InMemoryStorage an abstract DocumentBinaryStorage

[dbuthay]

.. and implementing it in-memory ..

This PR separates InMemoryStorage in 2 classes:

• DocumentBinaryStorage: an storage agnostic class that basically Serializes / compresses Documents in an efficitent way
• InMemoryStorage: a subclass of the above, that uses a Map<String, byte[]> to store the values.

Other possible subclasses extending DocumentBinaryStorage would be a krati storage, maybe a BerkeleyDB storage, or even a LevelDB storage.

BTW, I already have a working Krati version based on this branch on dbuthay/indextank-engine@2718a9efbdd8fa225e017d89f36feb8c4fe3458b