http-api: T6069: fix allocation outside of thread lock

[jestabro]

Change Summary

This was a dumb regression that led to a segfault under concurrent configuration requests; it has a simple fix of moving the instantiation of Config (hence configtree allocation) under the thread lock.

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/T6069

Related PR(s)

Component(s) name

Proposed changes

How to test

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

[c-po]

Should this be backported to sagitta?

[jestabro]

@Mergifyio backport sagitta

[mergify]

backport sagitta

🟠 Waiting for conditions to match

• merged [📌 backport requirement]

[mergify]

backport sagitta

✅ Backports have been created

• #3094 http-api: T6069: fix allocation outside of thread lock (backport #3089) has been created for branch sagitta