Data corruption can happen when components are multi-threaded because of non thread-safe serializer

OutputCollector is not thread-safe in 2.x.

It can cause data corruption if multiple threads in the same executor calls OutputCollector to emit data at the same time:

1. Every executor has an instance of ExecutorTransfer
https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/executor/Executor.java#L146

2. Every ExecutorTransfer has its own serializer

https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/executor/ExecutorTransfer.java#L44

3. Every executor has its own outputCollector

https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/executor/bolt/BoltExecutor.java#L146-L147

4. When outputCollector is called to emit to remote workers, it uses ExecutorTransfer to transfer data

https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/executor/ExecutorTransfer.java#L66

5. which will try to serialize data

https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/daemon/worker/WorkerTransfer.java#L116

6. But serializer is not thread-safe

https://github.com/apache/storm/blob/00f48d60e75b28e11a887baba02dc77876b2bb3d/storm-client/src/jvm/org/apache/storm/serialization/KryoTupleSerializer.java#L33-L43

But in the doc, http://storm.apache.org/releases/2.1.0/Concepts.html, it says outputCollector is thread-safe.

Its perfectly fine to launch new threads in bolts that do processing asynchronously. OutputCollector is thread-safe and can be called at any time.

We should either fix it to make it thread-safe, or update the document to not mislead users