thrift 协议代码错误

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Describe the bug (描述bug)
函数 WriteThriftMessageBegin
https://github.com/brpc/brpc/blob/master/src/brpc/policy/thrift_protocol.cpp#L118

填写 seq_id 的时候没有把指针转成 uint32_t *，导致 seq_id 不正确，java client 会报错：out of sequence response

To Reproduce (复现方法)
thrift server 不使用 ThriftFramedMessage 里的 _raw_instance（即不使用 Cast 函数），直接使用 butil::IOBuf body 时，能走到 WriteThriftMessageBegin 里。

Expected behavior (期望行为)

Versions (各种版本)
OS:
Compiler:
brpc:
protobuf:

Additional context/screenshots (更多上下文/截图)

static void
WriteThriftMessageBegin(char* buf,
                        const std::string& method_name,
                        ::apache::thrift::protocol::TMessageType mtype,
                        uint32_t seq_id) {
    char* p = buf;
    *(uint32_t*)p = htonl(THRIFT_HEAD_VERSION_1 | (((uint32_t)mtype) & 0x000000FF));
    p += 4;
    *(uint32_t*)p = htonl(method_name.size());
    p += 4;
    memcpy(p, method_name.data(), method_name.size());
    p += method_name.size();
    *p = htonl(seq_id);    // !!! 这里
}

[chenBright]

Closed this as completed in #2984 .