localThresholdMS should default to 15ms not 0ms

According to the server selection spec (https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#localthresholdms), localThresholdMS defaults to 15ms.  The Node driver defaults to 0ms.  

Acceptance Criteria

• Set the default for localThresholdMS to 15ms.
• Unskip the two failing server selection tests (Jira ticket linked with them)
• Look for default and configured localThreshold tests and implement if they don't exist
• Ensure the new default of 15 ms is respected