MULE-6959: Race condition creating MVELExpressionLanguage instances

_ Reverting previous commit
_ Adding MVELExpressionLanguageWrapper to take care of updating the DefaultExpressionManager.
(cherry picked from commit 5fd3e6e)
MULE-6959: Race condition creating MVELExpressionLanguage instances

_ Fixing license header
_ Moving test to a different package
(cherry picked from commit d1595fb)