Implement a new `read_at` method that will bind mount the tree of the
object to a specified location, instead of a temporary directory as
it done in the `read` method. Implement the latter via `read_at`.
Implement the corresponding methods for `Store{Client,Server}`. Since
the `ObjectStore.read_at` method will fail if the target directory
does not exist (or is of the wrong type), catch any exceptions in
the `StoreServer` and send those to the `StoreClient` via an `error`
entry.
This one is for David: also fix a missing blank line.
f8428e56e2