ensure proper shutdown of throttle (#4233)

arangod/RocksDBEngine/RocksDBEngine.cpp

@@ -607,6 +607,13 @@ void RocksDBEngine::unprepare() {
   }
 
   if (_db) {
+    // turn off RocksDBThrottle, and release our pointers to it
+    if (nullptr != _listener.get()) {
+      _listener->StopThread();
+      _listener.reset();
+      _options.listeners.clear();
+    } // if
+
     for (rocksdb::ColumnFamilyHandle* h : RocksDBColumnFamily::_allHandles) {
       _db->DestroyColumnFamilyHandle(h);
     }