Added FdbTransaction.GetAddressesForKeyAsync() (locality API)

- returns a string[] with the IPs of the storage nodes responsible for the key
- added a unit test
Added FdbKeyRange.FromPrefixInclusive ("key"..."key\xFF") and FdbKeySelectorPair.Create(FdbKeyRange) helpers

- also added a tr.GetRange(FdbKeyRange) override that will convert the range into a selector pair