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

This PR comprises the part of issue #499 regarding mongoBackend.
Actually, there are more changes:

```
   * - Reg1: CR: (E1,E2,E3) (A1,A2,A3)
   *         CR: (E1)       (A1,A4)
   * - Reg2: CR: (E2)       (A2, A3)
   * - Reg3: CR: (E1*)      (A1*)
   * - Reg4: CR: (E1**)     (A1)
```

versus

```
   * - Reg1: CR: (E1,E2,E3) (A1,A2,A3)
   *         CR: (E1)       (A1,A4)
   * - Reg2: CR: (E2)       (A2, A3)
   * - Reg3: CR: (E2*)      (A2*)
   * - Reg4: CR: (E3**)     (A2)
```

At the end, I will end with a non-linear function with ifs, etc. (non-linear functions are not good for unit test in general, as they make them less readable) so I think is prefereable to have two separated functions.

In my opinion, NTC