MULE-6800: Thread leak on Mule redeployments for embedded

_ Some changes on the log factory classes to enable better testing without involving a big refactoring. Both log factories are creating a LoggerReferenceHandler on the constructor. Wrapping that creation inside a factory method provides a way to override that on the tests. That is not the best approach, but refactoring factories and logger reference handler is out of scope.
Merge pull request #96 from mulesoft/MULE-6800

MULE-6800: Thread leak on Mule redeployments for embedded