Batch sync jobs

[sssoleileraaa]

Description

See parent issue #468

We want to wait before telling the user that a sync is finished until after everything in a sync batch has been downloaded and decrypted.

This means we need a way to indicate the end of a batch.

First idea: Create a SyncFinishedJob (or maybe call it BatchJob) that we add to the end of our queue after enqueing all new messages and replies. This job will give us a simple way to send the synced signal if it returns successfully, which it will always do if it's processed.

Second idea: We could create a parameter for DownloadJob that indicates whether or not the job is the last job to be processed in the sync. We could call it last_sync_job or something like that. When the job finishes, it can send this in its result.

More I think about it, the more I like the second idea.

[sssoleileraaa]

#468 and this issue are pretty much the same so closing