Task/392 service path rest of query context

### Description
Simplified the EntityId part of the query.
Also, "added" the ServicePath path for the query.
I think this PR worth a line in the CHANGES_NEXT_RELEASE file:

```
- Fix: entitiesQuery() method (used in queryContext and NGSI10 subscriptions) has been re-written, making it more clear and optimal (PR #405)
```
This would be an exception to the general rule of not adding in the changelog entries for modifications that doesn't affect to the user (an even more exceptional to include a reference to a PR :), but I think in this case we should do that. Although it seems that all works in the unit test and test harness with the new query schema, this "mark" in the changelog will ease locating the change if we found problems in the future.