Improvements for configurating dynamically

Motivation:
It seems that IoTDB server currently does not support dynamically configurating with session API or commands. If we want to hot modify the configuration of IoTDB, we can only edit the configuration files and execute `load configuration` command to achieve this goal.

Improvements:
We can add support for getting and setting single configuration value. For example, MySQL supports showing and dynamically modifying the configuration of server with query statements such as `SHOW variables` and `SET PERSIST ...`.

We might support the APIs in both session API and commands.

首先检查现有的 session API、命令接口，以及当前的配置文件和 `load configuration` 流程。定义如何通过两个 API 获取和更改单个配置值，然后验证动态 get/set 行为是否能够在不编辑配置文件的情况下保持一致地工作。

由索引模型根据 Issue 内容生成。