Clear counter when dhcp6relay init

[yaqiangz]

Fix issue sonic-net/sonic-buildimage#15047

Why I did it

When dhcp6relay startup, it would clear and reset the counter in DHCP_RELAY table, and it would cause that counter data of deleted Vlan wouldn't be clear.

How I did it

Clear all counter when dhcp6relay init

How I verify it

1. UT
2. Install new dhcp6relay in testbed
   2.1. Add Vlan2000,
   2.2. Send dhcpv6 packets to Vlan2000, confirm that counter has been updated
   2.3 reload config without Vlan2000, confirm that counter data of Vlan2000 has been deleted

admin@bjw-can-2700-3:~$ sonic-db-cli STATE_DB hgetall "DHCPv6_COUNTER_TABLE|Vlan1000"
{'Unknown': '0', 'Solicit': '1', 'Advertise': '0', 'Request': '0', 'Confirm': '0', 'Renew': '0', 'Rebind': '0', 'Reply': '0', 'Release': '0', 'Decline': '0', 'Reconfigure': '0', 'Information-Request': '0', 'Relay-Forward': '4', 'Relay-Reply': '0', 'Malformed': '0'}
admin@bjw-can-2700-3:~$ sonic-db-cli STATE_DB hgetall "DHCPv6_COUNTER_TABLE|Vlan2000"
{'Unknown': '0', 'Solicit': '3', 'Advertise': '0', 'Request': '0', 'Confirm': '0', 'Renew': '0', 'Rebind': '0', 'Reply': '0', 'Release': '0', 'Decline': '0', 'Reconfigure': '0', 'Information-Request': '0', 'Relay-Forward': '3', 'Relay-Reply': '0', 'Malformed': '0'}
admin@bjw-can-2700-3:~$ sudo config reload -y
Acquired lock on /etc/sonic/reload.lock
Disabling container monitoring ...
Stopping SONiC target ...
Running command: /usr/local/bin/sonic-cfggen -j /etc/sonic/init_cfg.json -j /etc/sonic/config_db.json --write-to-db
Running command: /usr/local/bin/db_migrator.py -o migrate
Running command: /usr/local/bin/sonic-cfggen -d -y /etc/sonic/sonic_version.yml -t /usr/share/sonic/templates/sonic-environment.j2,/etc/sonic/sonic-environment
Restarting SONiC target ...
Enabling container monitoring ...
Reloading Monit configuration ...
Reinitializing monit daemon
Released lock on /etc/sonic/reload.lock
admin@bjw-can-2700-3:~$ sonic-db-cli STATE_DB hgetall "DHCPv6_COUNTER_TABLE|Vlan1000"
{'Unknown': '0', 'Solicit': '0', 'Advertise': '0', 'Request': '0', 'Confirm': '0', 'Renew': '0', 'Rebind': '0', 'Reply': '0', 'Release': '0', 'Decline': '0', 'Reconfigure': '0', 'Information-Request': '0', 'Relay-Forward': '0', 'Relay-Reply': '0', 'Malformed': '0'}
admin@bjw-can-2700-3:~$ sonic-db-cli STATE_DB hgetall "DHCPv6_COUNTER_TABLE|Vlan2000"
{}
admin@bjw-can-2700-3:~$ 

[yaqiangz]

Hi @yxieca could you please help to review this PR?

[mssonicbld]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[yaqiangz]

Hi @bingwang-ms could you please help to approve the backport request?