mongoBackend "redirect" to context provideres for queryContext and updateContext operations

This PR comprises the part of issue #499 regarding mongoBackend.
There is no local copy for E1/A4 (note that we don't even initializie the entities connection in the prepareDatabase function). Note this is an UPDATE so it cannot create new context in the local copy: it's always proxied to a PA (or return in an 404 Not Context Element Found error).

By the way, I should include a check in the test with "entities count equal to 0" to ensure that no local context in created (although the usage of UPDATE should ensure this, this way the tests would be stronger).