Change PubSub::listen() exist on both SIGTERM and SIGINT and add Python signal handler support.

[liuh-80]

Why I did it

Fix hostcfgd can't terminate by SIGTERM and SIGINT issue:
https://github.com/Azure/sonic-swss-common/issues/603

PubSub::listen() need exist when receive SIGTERM and SIGINT.

How I did it

Change code to exist when receive SIGTERM and SIGINT.
Add python signal handler support.

How to verify it

1. manually test.
2. Pass all test case.

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

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Link to config_db schema for YANG module changes

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

[liuh-80]

Will improve code coverage by add new UT and update later.

[liuh-80]

Example code:

import signal
import os
import time
import threading

from swsscommon.signal import RegisterSignal

def dummy_signal_handler(signum, frame):
print("SIGNAL {} happen".format(signum))

RegisterSignal(signal.SIGINT, dummy_signal_handler) <== register python signal handler to swss-common

from swsscommon.swsscommon import ConfigDBConnector

conn = ConfigDBConnector()
conn.connect()
conn.subscribe('A', lambda a: None)
conn.listen()
^CSIGNAL 2 happen

[liuh-80]

Code change in this PR need test with another PR for buildimage repo not break by this PR: sonic-net/sonic-buildimage#11100

[liuh-80]

Closed because steppan's PR have a better solution: https://github.com/Azure/sonic-swss-common/pull/624/files