Add ability to retrieve runtime configuration from DB

When we create a Libpod database, we store a number of runtime
configuration fields in it. If we can retrieve those, we can use
them to configure the runtime to match the DB instead of inbuilt
defaults, helping to ensure that we don't error in cases where
our compiled-in defaults changed.

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/in_memory_state.go

@@ -73,6 +73,11 @@ func (s *InMemoryState) Refresh() error {
 	return nil
 }
 
+// GetDBConfig is not implemented for the in-memory state
+func (s *InMemoryState) GetDBConfig() (*DBConfig, error) {
+	return nil, ErrNotImplemented
+}
+
 // SetNamespace sets the namespace for container and pod retrieval.
 func (s *InMemoryState) SetNamespace(ns string) error {
 	s.namespace = ns