Fix incorrect sequence id setting in thrift message begin

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

brpc/src/brpc/policy/thrift_protocol.cpp

Lines 112 to 123 in c24e641

	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);

p is char*, so *p = htonl(seq_id); can not set seq_id(a uint32_t variable) to buf.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
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