code: public DataSourceContextHolder useSlave(boolean useSlave) {
  		DataSourceContextVals vals = contextVals.get();
  		if (vals == null)
  			vals = new DataSourceContextVals();
  		vals.userSlave = useSlave;
  		contextVals.set(vals);
  		return this;
  	}
docs: 设置是否使用从库
  
  @param useSlave