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.

Start by reviewing the existing session API, command interfaces, and the current configuration-file plus `load configuration` flow. Define how single configuration values should be retrieved and changed through both APIs, then verify that dynamic get/set behavior works consistently without editing configuration files.

Written by the indexing model from the issue text.