多线程Write()支持

[gaodq]

floyd/floyd/src/floyd_impl.cc

Lines 385 to 391 in 01a6979

	CmdRequest cmd;
	BuildWriteRequest(key, value, &cmd);
	CmdResponse response;
	Status s = DoCommand(cmd, &response);
	if (!s.ok()) {
	return s;
	}

这里是不是可以支持一下多线程写，像leveldb那样的多个写合并到一个中

[baotiao]

we have support batch writes when we writing log entries and when apply the log entries to state machine

[gaodq]

floyd/floyd/src/floyd_impl.cc

Lines 608 to 618 in 01a6979

	Status FloydImpl::ExecuteCommand(const CmdRequest& request,
	CmdResponse *response) {
	// Append entry local
	std::vector<const Entry*> entries;
	Entry entry;
	BuildLogEntry(request, context_->current_term, &entry);
	entries.push_back(&entry);
	response->set_type(request.type());
	response->set_code(StatusCode::kError);
	uint64_t last_log_index = raft_log_->Append(entries);

I mean before writing to log, entries at L618 will always contain only one item. I suppose we can merge multi threads' write operation into entries.