Added FdbKeyRange.FromKey(Slice)

- Similar to FromPrefix, but returns the range (key, key + '0')
- Creates a range that only select the specified key
Added comments
Added a trace when unhandled exceptions occur
Fixed AddConflictRange, and added Add(Read|Write)Conflict(Range|Key) overrides

- Moved AddConflictRange to IFdbReadTransaction
- Native takes a FdbKeyRange
- Added extension methods that takes slices and tuples
- Added unit tests for Read and Write conflicts
Simplified cleanup logic in Futures

- TryCleanup doesn't care if it is called from a callback