[counter] Clear counter table when dhcp6relay init

[yaqiangz]

Why I did it

Fix issue sonic-net/sonic-buildimage#15047
dhcpv6 counter didn't restore after config reload

Work item tracking

• Microsoft ADO (number only): 26065484

How I did it

1. Clear all counters when dhcp6relay start and add related ut
2. Remove clear counter in initialize_counter since they are cleared in previous

How to verify it

1. UT passed
2. Build debian packet and install in DUT do manually testing
3. Run dhcp_relay test, all passed

============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.1.3, pluggy-1.2.0
ansible: 2.9.27
rootdir: /var/src/sonic-mgmt-int/tests, configfile: pytest.ini
plugins: html-3.2.0, forked-1.6.0, ansible-3.1.5, xdist-1.28.0, allure-pytest-2.8.22, repeat-0.9.1, metadata-3.0.0
collected 22 items

dhcp_relay/test_dhcp_pkt_fwd.py::TestDhcpPktFwd::testDhcpPacketForwarding[pktInfo0] PASSED [  4%]
dhcp_relay/test_dhcp_pkt_fwd.py::TestDhcpPktFwd::testDhcpPacketForwarding[pktInfo1] PASSED [  9%]
dhcp_relay/test_dhcp_pkt_fwd.py::TestDhcpPktFwd::testDhcpPacketForwarding[pktInfo2] PASSED [ 13%]
dhcp_relay/test_dhcp_pkt_fwd.py::TestDhcpPktFwd::testDhcpPacketForwarding[pktInfo3] PASSED [ 18%]
dhcp_relay/test_dhcp_relay.py::test_interface_binding PASSED             [ 22%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_default[single] PASSED    [ 27%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_after_link_flap[single] PASSED [ 31%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_start_with_uplinks_down[single] PASSED [ 36%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_unicast_mac[single] PASSED [ 40%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_random_sport[single] PASSED [ 45%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_counter[single] PASSED    [ 50%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_default[dual] SKIPPED     [ 54%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_after_link_flap[dual] SKIPPED [ 59%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_start_with_uplinks_down[dual] SKIPPED [ 63%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_unicast_mac[dual] SKIPPED [ 68%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_random_sport[dual] SKIPPED [ 72%]
dhcp_relay/test_dhcp_relay.py::test_dhcp_relay_counter[dual] SKIPPED     [ 77%]
dhcp_relay/test_dhcpv6_relay.py::test_interface_binding PASSED           [ 81%]
dhcp_relay/test_dhcpv6_relay.py::test_dhcpv6_relay_counter PASSED        [ 86%]
dhcp_relay/test_dhcpv6_relay.py::test_dhcp_relay_default PASSED          [ 90%]
dhcp_relay/test_dhcpv6_relay.py::test_dhcp_relay_after_link_flap PASSED  [ 95%]
dhcp_relay/test_dhcpv6_relay.py::test_dhcp_relay_start_with_uplinks_down PASSED [100%]INFO:root:Can not get Allure report URL. Please check logs

[yaqiangz]

@kellyyeh Could you help to review and merge this PR?

[dgsudharsan]

@yaqiangz @yxieca Can you please update dhcp-relay submodule in 202205 so this fix will be included?