What is the purpose of the change

Resolve concurrent bug (or NullPointerException) in DubboProtocol.java

Brief changelog

The statement "locks.get(key)" will get NULL after "locks.remove(key);", and then "synchronized(null)" will throw NullPointerException. Obviously, this two statement, "locks.putIfAbsent(key, new Object());" and "locks.get(key)", are not an atomic operation.

I can't find a better way to reduce lock granularity, maybe we have to sacrifice some performance to ensure safety here.

Verifying this change

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