[IOTDB-1311] CoAP Protocol Support

The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things.

Besides MQTT, as the DataBase for IoT, IoTDB should support CoAP protocol to meet many customer's requirements.

We should design a plugin mechanism to support MQTT, CoAP etc.

首先，审查 IoTDB 当前如何支持 MQTT，以及协议集成是如何组织的。然后，确定针对 MQTT 和 CoAP 所提议的插件机制的范围。完成的标准是形成一份达成一致的设计，并实现满足所述客户需求的 CoAP 支持。

由索引模型根据 Issue 内容生成。