peek for Fermin

### This is not a PR
Just used to interchange info.
Will be removed after fulfilling its purpose
If we want to limit the number of service paths to 1 in subscribeContext, the checking should be done in this "flow". Not at mongoBackend, but in the previous layer (parsing).

Probably the same applies to updateContextSubscription. Note that different from entities, which are identified by the triplet <id, type, sp>, subscriptions are identified by just id (generated by mongo). Thus, in that way, the Fiware-ServicePath in the updateContextSubscription could be used as the new servicePath to use in the subscription (i.e. an easy way of implementing service path updating, things that is not so easy in the case of entities).