Rewrite NettyRpcServer to decode rpc request with netty handler

This is a follow on issue of HBASE-26708.

The memory leaks happens in the saslReadAndProcess method. It does not follow a typical netty style so it is more likely to introduce bugs.

Since we agree that NettyRpcServer is the future and SimpleRpcServer should be deprecated and finally removed, let's first implement dedicated rpc request decoder for netty.