Release buffer redundantly in ServerHandler

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Fluss version

0.6.0 (latest release)

### Please describe the bug 🐞

Currenly, fluss release buffer in server places:
1. com.alibaba.fluss.rpc.netty.client.NettyClientHandler#channelRead
2. com.alibaba.fluss.rpc.netty.server.NettyServerHandler#sendResponse
3. com.alibaba.fluss.rpc.netty.server.RequestProcessor#run

There is no guarantee that the buffer of a request is only release once.

### Solution

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by reading NettyClientHandler#channelRead, NettyServerHandler#sendResponse, and RequestProcessor#run. Trace how a request buffer moves through these three release points and verify the ownership behavior with the relevant RPC tests. Done means a request buffer is released exactly once.

Written by the indexing model from the issue text.