All states are lockers

Since moving to the new backends, all states (except InmemState) are
Lockers. Add the methods to the State interface to remove a heap of
assertion checks.

state/inmem.go

@@ -26,3 +26,11 @@ func (s *InmemState) WriteState(state *terraform.State) error {
 func (s *InmemState) PersistState() error {
 	return nil
 }
+
+func (s *InmemState) Lock(*LockInfo) (string, error) {
+	return "", nil
+}
+
+func (s *InmemState) Unlock(string) error {
+	return nil
+}

state/state.go

@@ -28,6 +28,7 @@ type State interface {
 	StateWriter
 	StateRefresher
 	StatePersister
+	Locker
 }
 
 // StateReader is the interface for things that can return a state. Retrieving