Don't blindly join rooms

[turt2live]

This appears to be causing issues with synapse:

• workers stop working after elevated traffic matrix-org/synapse#2738 (comment)
• /rooms/join emits a join event even if the user is already in the room matrix-org/synapse#3337

Instead, let's try doing a directory lookup to resolve the room ID. From the room ID, increment the link count or join the room if we haven't already (analyze /joined_rooms maybe?).