Fix MagicFolderConfig.store_local_snapshot to use transactions correctly.

[tomprince]

It uses the with_cursor decorator, but calls it self recursively. If I am reading the sqlite docs and sqlite module docs correctly, this won't do the right thing, as the transaction type used is not recursive.

It looks like this is the only function that runs into this issue, but it would be good to make with_cursor resilient to this kind of error as well.

[tomprince]

It turns out that it does use transaction incorrectly, as the recursive call will cause the transaction in the parent call to be committed, without having the parent links written. If we crash immediately after that commit, we won't be able to load snapshots for that file, since there will be two leaf snapshots, and we will hit this exception.

There is also a transaction issue in MagicFolderConfig.get_local_snapshot accessing the .author property, which is itself decorated with with_cursor. However, this case is less of an issue since they are both read-only transactions.