[entropy.db] EntropyBaseRepository: turn lock methods into no-op if direct mode

Now that the repository lock is reentrant, it's good to have the
methods take into account direct mode as well. In direct mode,
we explicitly don't want to deal with any kind of locking, because
we accept to manipulate stale data. In order to hide locking code
from the outside and have it transparently managed inside entropy.*
methods, we must respect requests made in direct mode.