Module cleanup - don't hold on to stuff after the command completes.

--
MOS_MIGRATED_REVID=120111522
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java b/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
index de79f05..73ba2f2 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BuildSummaryStatsModule.java
@@ -47,6 +47,13 @@
     eventBus.register(this);
   }
 
+  @Override
+  public void afterCommand() {
+    this.criticalPathComputer = null;
+    this.eventBus = null;
+    this.reporter = null;
+  }
+
   @Subscribe
   public void executionPhaseStarting(ExecutionStartingEvent event) {
     criticalPathComputer = new SimpleCriticalPathComputer(BlazeClock.instance());