Subspaces: removed the IFdbSubspace interface (not used), moved most of the non-essential methods of FdbSubspace into extension methods, and cleaned up the API

* FdbSubspace only has the base methods (Key, Contains, Concat, Extract, ToRange, ...)
* All others helper methods (and overrides) are now extensions methods (some may be moved back as member methods if needed)
* Removed Merge(...) that was a duplicate of ConcatRange(...)
* Renamed Append(object[]) into AppendBoxed(object[])
* FdbSubspace now implements IEquatable<FdbSubspace> and IComparable<FdbSubspace>
* FdbDatabase (and filters) now implements IFdbKey instead of IFdbSubspace
* Added missing overrides to some methods, fixed comments, ...
* Added warnings where we have "params object[] ..." because it can be dangerous
* The changes should be source-compatible, except subspace.Merge(...) that must be changed to subspace.ConcatRange(...)