Bulk: started refactoring some of the Fdb.Bulk methods to take a WriteOptions class

- Fixed bug in ForEachAsync() where the GenerationTimer was not restarted properly after the first retry.
- WriteOptions will allow adding more knobs to tune the bulk retry loops in the future without breaking API
- Breaking change on Fdb.Bulk.WriteAsync(..., IProgress<T> progress, ...)  which is now passed via the WriteOptions class
- Cleaned up loging in some of the unit tests
Updated Json.NET to 6.0.6 and fixed the bad HintPath which broke the build in TeamCity
Bulk: don't call Trace.WriteLine in release builds !
Unit Tests: added a Log(..) helper on FdbTest base class

- always use InvariantCulture for formatting logs, for non-english cultures