Fetch positions to ignore from config collection

[arturgontijo]

This PR adds the logic to ignore patterns that have "*" at one or more positions (eg. composite_key[position] == "*").

Currently the list of positions to be ignored must be stored in a MongoDB document inside a new "config" collection, eg:

{
   "_id": "1",
    "positions_to_ignore": [0, 2]
}

This means that we must ignore "*" at the positions 0 and 2.
Heads up that the _id value is hardcoded for now, but we can pass it via parameter.

TODO:
One important feature that is still not implemented would be a way to check if a given handle is found in a given position.
As an example, a user would want to store only patterns that have EVALUATION, PREDICATE or CONCEPT in the position 0.