MemoryDB: tweaks to allow the empty key
MemoryDB: continued implementation for the snapshot file reader

- Switched to unsigned integers for offsets and sizes
- Fixed Win32SnapshotFile so that we can actually read from it
- Fixed field size errors when reading or writing
- Implemented reading JumpTables and Levels
- Added LevelWriter helper class that allows streaming data into a level
- Reader should probably be redsigned to use and USliceReader over a memory mapped file