Ensure a 3rd party can't make us communicate with another party on their room

[andrevmatos]

Problem Definition

Out of #2501 #2302
Related: #473

A---B
 \ /
  C

Currently, the node joins any rooms it's invited to (to listen for possible partners messages), and verifies only the sender of the message to update the room it uses to comunicate with that node. This allows an attacker C to invite us (A) to a room it is communicating with B, and with whom we also have a channel, which (upon seeing the first B's message) will make us update and start communicating with B on that room (which is owned/readable by C), possibly leaking sensible information once we get private/encrypted rooms.

We need to ensure the matrix invite system only accepts invites to secure (i.e: private) rooms, and only communicates/updates with that inviter node on this room.