Document sync process

[eloquence]

To better reason about UI & network improvements, it would be useful to have a quick write-up of what's involved in a sync operation with the server.

E.g., say that there are 300 sources with typically 3 messages, 3 files, and 3 replies on the server. Out of those 10 sources are completely new to the client.

What is the sequence of steps that the client performs:

• in order to get the initial metadata
• in order to download messages and replies it doesn't have yet?

When does the UI currently get updated during those operations?

[sssoleileraaa]

Here's a start: doc

And see issue #468 where I've also documented the sync process

[eloquence]

The doc is awesome, thank you. <3 Left a couple questions there.

[sssoleileraaa]

are you happy with closing this issue @eloquence ?

[eloquence]

Yes, we can close. Thanks again for putting this together. :)