Store reactions

[gferon]

Relating to #95, when we receive a reaction, we could either:

• modify the original stored message (or store the reactions aside somewhere)
• save the reaction without a reference to the original message
  and send back the result in the stream.

This will help apps not having to merge those events themselves when rendering.

[Schmiddiii]

I agree that this would be nice to have. Regarding the possible options:

modify the original stored message

I don't really think that would be nice, as the reaction-field for messages is for reacting for other messages and should probably not be abused for stroring reactions to the stored message. (Unless we want to introduce a completely new Message-type wrapping the DataMessage that will be stored (which may also be worth thinking about having nice wrappers instead of just the proto-things)).

store the reactions aside somewhere

I agree with that option the most. Maybe introduce a new tree for every thread mapping the message timestamp to a list of reactions or only one tree mapping a combination of thread and message timestamp to the list of reactions.

save the reaction without a reference to the original message and send back the result in the stream

Not sure what you exactly mean by that.