Don't handle subscribe message until we change the API to secure it

[vincentfretin]

Don't handle subscribe message until we change the API to secure it with JWT.
This API is not used currently by naf-janus-adapter, but this can still be used for eavesdropping if the UserId of a participant in a room can be easily known (not a random clientId), see my example mozilla#81 (comment)

This is a temporary PR for those using JWT to secure their rooms. I won't merge this.
I'll close this PR once I work on:

• Check security token in process_subscribe mozilla/janus-plugin-sfu#83
• Update api.md mozilla/janus-plugin-sfu#86

[vincentfretin]

I actually merged that in master. I'll revert the commit on master and enable again the subscribe message with proper security when I'll have the use case for it.