Shared db locking was used to avoid the case in which two different devices
try to store/modify remotelly stored secrets at the same time. We want to
avoid remote locks because of the problems they create, and prefer to crash
locally.

For the record, we are currently using the user's password to encrypt the
secrets stored in the server, and while we continue to do this we will have to
re-encrypt the secrets and update the remote storage whenever the user changes
her password.

- Use dbsyncer (SQLCipherU1DBSync) instead of SQLCipherDatabase
  as only the first one supports multiple threads while syncing
  and is actually used by Soledad.sync