Motivation:
- The encoder and decoder should be closed right after the handler releases its resources.
- The clientPrefaceString is allocated in the constructor but releases in handlerRemoved.
If the handler is never added to the pipeline, the clientPrefaceString will never be
released.
Modifications:
- Call encoder.close() and decoder.close() on channelInactive.
- Release the clientPrefaceString on handlerRemoved.
Result:
- The encoder and decoder get closed right after the handler's resources are freed.
- It's easier to verify that the clientPrefaceString will also get released.
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