This refactors the store to make it composable with arbitrary mtl monad
stacks, with the added constraint that `addToStore` takes a filtering
fucntion `FilePath -> PathFilter -> m Bool` which is not
MonadBaseControl compatible, and cannot be lifted (the monad is in a
negative/contravariant position).

The solution involves a RemoteStoreT transformer, a MonadRemoteStore
monad and still lacks a proper generic MonadStore which I would like to
make generic across all the store implementations (in-memeory /
read-only / remote daemon / etc.)