Cleanup and pep8 fix.
Add splitted POST sync design docs (#5571).
Split sync in multiple POST requests in server (#5571).
Split sync in multiple POST requests in client (#5571).
Allow for interrupting and recovering sync (#5571).
Add sync status signals (#5517).
fix allow interrupt in target client
Properly raise when couch db does not exist.
Fix tests for new sync process.
Fix stuff from kali's review.
Turn SQLCipher.sync_state into a ClientSyncState instance.
Merge remote-tracking branch 'refs/remotes/drebs/feature/5517_split-sync-post' into develop