Shutdown digester thread pool when done.

Closes #89.

src/main/java/com/threerings/getdown/data/Digest.java

@@ -48,7 +48,8 @@ public class Digest
         throws IOException
     {
         // first compute the digests for all the resources in parallel
-        Executor exec = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
+        ExecutorService exec = Executors.newFixedThreadPool(
+            Runtime.getRuntime().availableProcessors());
         final Map<Resource, String> digests = new ConcurrentHashMap<>();
         final BlockingQueue<Object> completed = new LinkedBlockingQueue<>();
         final int fversion = version;
@@ -71,6 +72,9 @@ public class Digest
             });
         }
 
+        // queue a shutdown of the thread pool when the tasks are done
+        exec.shutdown();
+
         try {
             while (pending.size() > 0) {
                 Object done = completed.poll(600, TimeUnit.SECONDS);