conntrack-sync: T1244: Support for StartupResync in conntrackd (backport #3254)

[mergify]

Change Summary

Added option for enabling startup resync

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T1244

Related PR(s)

vyos/vyos-documentation#1352

Component(s) name

conntrack-sync

Proposed changes

How to test

set high-availability vrrp group internal interface 'eth0'
set high-availability vrrp group internal virtual-address 10.0.0.10/24 interface 'eth0'
set high-availability vrrp group internal vrid '36'
set high-availability vrrp sync-group syncgrp member 'internal'
set service conntrack-sync failover-mechanism vrrp sync-group 'syncgrp'
set service conntrack-sync interface eth0
set service conntrack-sync startup-resync
commit

vyos@vyos# cat /run/conntrackd/conntrackd.conf | grep StartupResync
        StartupResync on

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3254 done by [Mergify](https://mergify.com).