Fixed bug where an FdbFutureArray<T> would not get canceled when the callback fires
Sealed a few classes that should not be derived from.

- We can always unseal them later
Fixed FdbSubspace.PackBoxedRange(..) methods that were not using the subspace's prefix
Renamed FdbSubspace.Create(...) into .Append(...) for methods that were returning a new Tuple.

- subspace.Append(...) => a new tuple haning off the subspace
- FdbSubspace.Create(...) => a new subspace
Call ToString() on value types before passing them to String.Format
FdbStringIntern.Uid class should be sealed
Disable slow benchmark test on queues
Shortened "keyBytes" and "valueBytes" arguments to just "key" and "value"
Forgot to also update AddConflictRange(...) to take both keys as params in the interface

- overloads that take FdbKeyRanges are now extension methods
Made sure overload arguments names are the same as in the base class/interface