Fix: race condition on shutdown of zip extractor

src/main/java/com/sheepit/client/UnzipUtils.java

@@ -245,6 +245,17 @@ public class UnzipUtils {
 	private static void writeWait(WriteQueue queue, ExecutorService executor) throws InterruptedException {
 		queue.doneAdding = true;
 		executor.shutdown();
+		
+		try {
+			if (executor.awaitTermination(60, TimeUnit.MINUTES) == false) {
+				executor.shutdownNow();
+			}
+		}
+		catch (InterruptedException e) {
+			executor.shutdownNow();
+			Thread.currentThread().interrupt();
+		}
+		
 		//Encourage the JVM to release memory used while decompressing
 		GC_RUNNER.execute(System::gc);
 	}