add state.Locker interface

Changed from state.StateLocker to remove the stutter.

State implementations can provide Lock/Unlock methods to lock the state
file. Remote clients can also provide these same methods, which will be
called through remote.State.

state/state.go

@@ -40,3 +40,9 @@ type StateRefresher interface {
 type StatePersister interface {
 	PersistState() error
 }
+
+// Locker is implemented to lock state during command execution.
+type Locker interface {
+	Lock(reason string) error
+	Unlock() error
+}