Better support for transaction

• Enhancements

  • refactored event notification system
  • added support for retryable reads and writes
  • refactored the test cases
  • now using mtools for a MongoDB deployment in the travis ci environment
  • travis ci uses only the latest MongoDB version The failCommand
  • Session.commit_transaction returns now :ok or an error {:error, %Mongo.Error{}}

• Bugfixes

  • Using max_staleness_ms > 0 results in a crash
  • Read preferences are sent to mongos