Fix and isolate message receipts

[Sup3Legacy]

This PR should both :

• Fix Rate limiting on too many receipts #115. This issue originated in a small bug causing the client to repeatedly send empty ReceiptMessages.
• Isolate the handling of these receipts. They take the form of ReceiptEvents that can be created anywhere in the client and they are handled in a separate structure, stored in a set (idea from @boxdot). At regular interval (default 1 per s.; this rate can safely be increased), an event, if any, is sent.

Technical clarification : the events are sent grouped by Uuid and whether they are Delivered or Read, so the number of waiting events cannot exceed 2 * (number of active conversations).

Shortcomings :

• This handler does not get saved in the database. This means that closing the client while some receipt events are still waiting to get sent will result in them never getting sent at all.

[boxdot]

Very nice! Thank you. -- I have some comments.

[boxdot]

Thank you. I just have the last two comments about the entry api, and then we can merge.

[boxdot]

Thank you! Awesome work.