Add log configuration handler to handle log level change in CONFIG DB

[Junchao-Mellanox]

Why I did it

SONiC provides two Python logger implementations: sonic_py_common.logger.Logger and sonic_py_common.syslogger.SysLogger. Both of them do not provide the ability to change log level at real time. Sometimes, in order to get more debug information, developer has to manually change the log level in code on a running switch and restart the Python daemon. This is not convenient.

SONiC also provides a C/C++ logger implementation in sonic-platform-common.common.logger.cpp. This C/C++ logger implementation is also a wrapper of Linux standard syslog which is widely used by swss/syncd. It provides the ability to set log level on fly by starting a thread to listen to CONFIG DB LOGGER table change. SONiC infrastructure also provides the Python wrapper for sonic-platform-common.common.logger.cpp which is swsscommon.Logger. However, this logger implementation also has some drawbacks:

• swsscommon.Logger assumes redis DB is ready to connect. This is a valid assumption for swss/syncd. But it is not good for a Python logger implementation because some Python script may be called before redis server starting.
• swsscommon.Logger wraps Linux syslog which only support single log identifier for a daemon.
  So, swsscommon.Logger is not an option too.

This PR enhances sonic_py_common.logger.Logger and sonic_py_common.syslogger.SysLogger to allow user set log level at run time.

Work item tracking

• Microsoft ADO (number only):

How I did it

The PR introduces a logger configuration thread. This thread listens to redis LOGGER table change and set log level for each logger instance accordingly.

How to verify it

Full Unit test
Manual test
Regression test

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)