Added : IFdbKey serialization in FdbTuplePackers & FdbSubspace.Concat(IFdbKey)
Added comparison, equality and orther methods to FdbKeyRange

- FdbKeyRange now implements IEquatable<..> and IComparable<..>
- Added Merge(), Intersects() and Disjoint() methods
- Added unit tests
Added the SliceBuffer helper class

- Stores slices and key paris into a set of pages in memory
- Usefull for batching