T7980: cache running config on commit completion and add option --reload-active-config

[jestabro]

Change Summary

NOTE: requires vyos/vyos1x-config#55

Use the quick-loading 'internal' representation of the running config to reload on vyconfd (re)start with option --reload-active-config.

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)
• Other (please describe): add missing internal feature

Related Task(s)

Related PR(s)

vyos/vyos1x-config#55
vyos/vyos-1x#4827

Proposed changes

How to test

Checklist:

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

[dmbaturin]

Config caching and the restartability that comes with it are some of the long-awaited vyconf features.

A small note is that in the future we'll almost certainly want read and write failures in vyconf very loud — potentially, blocking all config operations, since if it can't write its cache, things in the system have gone in a really wrong direction and it's unlikely that any commit can succeed in that state.