Room can't be destroyed inside Room::message_received

[inetic]

It causes double free of Conversations inside ConversationList.

We do mention that we can't call any room modifiers from inside Room's callbacks, but this is a bit different as the call to Room::message_received often results ConversationInterface callback being called. Implying that we can't destroy Room from inside those callbacks neither.

Possible solutions:

• Fix it such that Room can be destroyed from inside ConversationInterface callbacks
• Update documentation stating that it can't be done.