Core API: Added Fdb.UseApiVersion(...) and added more explicit error message when SelectApiVersion fails

- People can ask for a specific API version by calling UseApiVersion(..) before start. If not, uses the default API version target (currently 200)
- If API select fail, error message will contains details on the min/max versions supported by the binding.
Build script: set NUnit timeout to 10 minutes, and kill nunit-agent.exe when its done
Marked some unit test as LongRunning, fixed timeout for nunit runner