Store non-encrypted docs in the sync db (#5895).
Insert received docs in sync db after decryption (#5895).
Store all received docs in sync db (#5895).
On sync, fetch all docs before decrypting.
Allow deferred decryption without deferred encryption.
Avoid deadlocks when cancelling failed sync threads.
Make sync database multiprocessing-safe.
Refactor secrets out of main soledad client class.
Fix tests after many changes in client.
Add instructions for closing SQLCipher db on docstrings.
Store decrypted storage secret in memory.
Add salt for sync sb key derivation.
Protect sync db with a password.
Delete the received docs from sync db before starting a new sync.
Fix bits from pullreq review.