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Merge branch 'release-0.5.2'
bump requirements to latest leap.common

this is needed due to the introduction of new events.
Merge remote-tracking branch 'refs/remotes/kali/bump-requirements' into develop
Merge branch 'master' into develop
Decouple sync from actual encryption/decryption (#5326).
Split sync_exchange into many requests (#5517).
Split tests and fix for new sync (#4710).
Add changes files.
Properly stop sync and cancel threads on fail.
Update target sync with sequential info (#5869).
Fix sqlcipher db syncing property.
Stop the target when exceptions occur.
Fix tests according to modified sync.
Add spam script for profiling.
Add doc for syncers dictionary.
Fix common exception handling gotchas.
Stop syncers when closing soledad.
Close all connections after syncing (#5518).
Fix recovery from failed sync.