Refactor message data & persistence model

[janherich]

User Story

As a developer, I want to have single place in app, where all data uniquely tied to particular message are held, I also want to have only one (clear) way how to save/update/retrieve message data in/from realm.

Description

Type: Problem

Summary: Currently, we have message data all over the place in app-db and realm. It's directly attached as an ordered sequence under [:chats chat-id :messages] key, then we have some data related to messages indexed by message id in [:message-data :preview], [:message-data :short-preview], [:message-data :status] and soon also in [:handler-data] and [:message-id->pending-messages].
It's even worse for the realm persistence API, where something is held in message objects, something not (handler-data, pending-messages).
Every-time when we need to have something message-related with random access by message-id, we invent a new place in app-db where we put it, then we have to keep track of it in subscriptions, when updating messages, write boilerplate code for realm persistence, etc.

Solution

Summary: We well move everything message related under [:message-by-id] (or [:id->message]) path in app-db. We will rework our realm schema, and persistence functions, eliminate boilerplate and use only handful of message related functions to do everything in regard of message persistence.
Under [:chats chat-id :messages] we will have (ordered) vector of message-references, such as:

[[:message-by-id "id1"]
 [:message-by-id "id2"]
 [:message-by-id "id3"]]

As mentioned, ordering of the messages will be maintained by ordering of message references (responsibility of db functions updating [:chats chat-id :messages] data) and it will be job of messages subscription(s) to use the references to look-up actual message objects and return it as sequence to view layer.