Fixed bad #ifs for .NET 4.0 vs 4.5
Removed unused methods in Slice, FdbKey and FdbKeyRange, added more test and cleanup

- FdbKey.Ascii/Unicode/Binary are redundant with Slice.FromAscii/Unicode
- Removed FdbKeyRange.All and None, replaced with Empty (default value)
- Validate the arguments better in Slice.Create(...) instead of returning Empty/Nil
- Added a Slice.Random that takes a crypto PRNG
- Removed ToAsci(..,..)/ToUnicode(..,..) that can be replace with  Substring(...).ToAscii()/ToUnicode()
- Some cleanup in Fdb.Errors helpers
- Augmented test coverage on core types
Merge branch 'master' of github.com:Doxense/foundationdb-dotnet-client