labels: allow restoring cache-map

E.g. with unexpected termination for mu it is possible that the
labels-cache (i.e., the one that is used for auto-completion) gets disconnected
from reality.

Add a --restore option to `mu label list` to restore the actual labels from the
labels seen in the store.

lib/mu-store.cc

@@ -694,6 +694,14 @@ Store::clear_labels(Message& message)
 	return Ok(std::move(updates));
 }
 
+Result<void>
+Store::restore_label_map()
+{
+	std::unique_lock lock{priv_->lock_};
+
+	return priv_->labels_cache_.restore(*this);
+}
+
 LabelsCache::Map
 Store::label_map() const
 {