Improve the error management in order to avoid DB corruption in case the process is killed while encrypting the DB.
Changes:
Use sqlcipher_export instead of rekey to change the DB password. The advantage is that sqlcipher_export will operate on a new DB file and we don't need to modify the current account unless the export is successful.
Keeping the rekey requires to create a DB copy before trying to re-encrypt the DB, but the DB copy is risky in case the DB file changes wile the copy is in progress. It could also lead to DB corruption.
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