Fix race in API related to LoadComplete()

[smira]

Fixes #686

Description of the Change

LoadComplete() modifies object, so it would cause issues if it runs
concurrently with other methods. Uprage mutex locks to write
locks when LoadComplete() is being used.

This reduces concurrency of some API calls but eliminates data races

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS