* Fetch conversations once, clean up ConversationController API
Race conditions around re-fetching have caused some problems recently,
so this removes the need to re-fetch conversations. They are fetched
once or saved once, and that is it. All interaction goes through the
ConversationController, which is the central source of truth.
We have two rules for Conversations:
1. If a conversation is in the ConversationController it doesn't need
to be fetched, but its initial fetch/save might be in progress. You
can wait for that fetch/save with conversation.initialPromise.
2. If a conversation is not already in the ConversationController, it's
not yet in the database. It needs to be added to the
ConversationController and saved to the database.
FREEBIE
* Remove Conversation.fetch() call in Message.handleDataMessage()
FREEBIE
* ConversationController.API cleanup: Fix two missing spots
FREEBIE
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