ARTEMIS-1852 PageCursorProvider is leaking cleanup tasks while stopping

PageCursorProviderImpl is not handling any pending cleanup tasks
on stop, leaving paging enabled due to the remaining pages to be
cleared up.
PagingStoreImpl is responsible to trigger the flushing of pending
tasks on PageCursorProviderImpl before stopping it and to try to
execute any remaining tasks on the owned common executor, before
shutting it down.
It fixes testTopicsWithNonDurableSubscription.

artemis-server/src/main/java/org/apache/activemq/artemis/core/paging/cursor/impl/PageCursorProviderImpl.java

@@ -239,8 +239,10 @@ public class PageCursorProviderImpl implements PageCursorProvider {
       for (PageSubscription cursor : activeCursors.values()) {
          cursor.stop();
       }
-
-      executor.shutdownNow();
+      final int pendingCleanupTasks = scheduledCleanup.get();
+      if (pendingCleanupTasks > 0) {
+         logger.tracef("Stopping with %d cleanup tasks to be completed yet", pendingCleanupTasks);
+      }
    }
 
    private void waitForFuture() {

artemis-server/src/main/java/org/apache/activemq/artemis/core/paging/impl/PagingStoreImpl.java

@@ -348,11 +348,23 @@ public class PagingStoreImpl implements PagingStore {
    @Override
    public synchronized void stop() throws Exception {
       if (running) {
+         cursorProvider.flushExecutors();
          cursorProvider.stop();
 
-         running = false;
+         final List<Runnable> pendingTasks = new ArrayList<>();
+         final int pendingTasksWhileShuttingDown = executor.shutdownNow(pendingTasks::add);
+         if (pendingTasksWhileShuttingDown > 0) {
+            logger.tracef("Try executing %d pending tasks on stop", pendingTasksWhileShuttingDown);
+            for (Runnable pendingTask : pendingTasks) {
+               try {
+                  pendingTask.run();
+               } catch (Throwable t) {
+                  logger.warn("Error while executing a pending task on shutdown", t);
+               }
+            }
+         }
 
-         executor.shutdownNow();
+         running = false;
 
          if (currentPage != null) {
             currentPage.close(false);