[actor shutdown] shutdown hooks (#74)

### Summary

Add two hooks on the `Actor` type, `before_stop` and `after_stop`. These
methods allow final cleanup code and message-sending during the shutdown
process.

### Motivation

Part of #29.

Actors need a mechanism to perform final cleanup operations on shutdown.

### Test Plan

TBD. Further evolution on the shutdown process will determine the
overall testing strategy.

src/actor/actor.rs

@@ -33,6 +33,24 @@ pub trait Actor: Send {
     /// Receive a message. This is the primary method for handling messages and is called
     /// for every message received by the actor.
     fn receive(&mut self, ctx: Context, msg: Box<dyn Message>);
+
+    /// Hook called after shutdown has been initiated, but before the actor has been removed
+    /// from the executor. This can be used to send any final messages before the actor is
+    /// stopped.
+    ///
+    /// Cleanup may also happen in this method, but may also happen in the [`Actor::after_stop`]
+    /// method.
+    ///
+    /// __Note:__ When an actor is stopped, new messages may not be received. So while
+    /// messages can be sent, no reply will be able to be processed.
+    fn before_stop(&mut self, _ctx: Context) {}
+
+    /// Hook called after the actor has been shutdown and removed from the executor. This is
+    /// the final chance to cleanup/close any resources before the actor is dropped.
+    ///
+    /// __Note:__ Messages can no longer be sent from this method. See [`Actor::before_stop`]
+    /// for the last chance to send messages.
+    fn after_stop(&mut self) {}
 }
 
 /// ActorInit defines a method of construction for an actor that takes an initialization

src/executor/thread_executor.rs

@@ -100,8 +100,9 @@ impl Executor for ThreadExecutor {
                     }
                     ExecutorCommands::ShutdownActor(address) => {
                         let cell = self.actor_cells.get_mut(&address.uri).unwrap();
-                        cell.set_shutdown()
-                        // TODO: Call on_shutdown hook for actor
+                        cell.set_shutdown();
+                        cell.actor
+                            .before_stop(context!(self, cell, SenderType::System));
                     }
                     ExecutorCommands::Shutdown => {
                         info!("received shutdown command");