Fix licenses

Summary:
There are two types of licenses used in folly, only one of
which is accepted by the linter.

This diff changes the license notices in pre-existing folly files!
Many folly/io/async/* files have the second type of license, but
without a Copyright notice. I have added copyright notices to these
files.

I have also added a compliant notice to
folly/test/function_benchmark/benchmark_impl.h, which was the sole file
in folly/test/function_benchmark/ that didn't have a standard license.

Test Plan:
The changes to folly are comment only.

Run all of folly against the linter, see no more license errors.

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, sdwilsh, njormrod, folly-diffs@, sjenkins

FB internal diff: D1648489

Tasks: 5486739

Signature: t1:1648489:1415035522:3d8bd9611eb7c7117b70d5e7f68de5768639a727

folly/io/async/AsyncTimeout.cpp

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/AsyncTimeout.h

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/EventHandler.cpp

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/EventHandler.h

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/EventUtil.h

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/HHWheelTimer.cpp

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/Request.cpp

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/Request.h

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/io/async/TimeoutManager.h

 /*
+ * Copyright 2014 Facebook, Inc.
+ *
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements. See the NOTICE file
  * distributed with this work for additional information

folly/test/function_benchmark/benchmark_impl.h

-// Copyright 2004-present Facebook.  All rights reserved.
+/*
+ * Copyright 2014 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 #ifndef BENCHMARK_IMPL_H_
 #define BENCHMARK_IMPL_H_