When migrating from a multi-state backend to a single-state backend, we
have to ensure that our locally configured environment is changed back
to "default", or we won't be able to access the new backend.
During backend initialization, especially during a migration, there is a
chance that an existing state could be overwritten.
Attempt to get a locks when writing the new state. It would be nice to
always have a lock when reading the states, but the recursive structure
of the Meta.Backend config functions makes that quite complex.
