Move _active_transitions from migration_plan to load_balancer class
as a member variable. This is cleaner since there's no concurrent
make_plan() calls on the same load_balancer instance.
Changes:
- Removed _active_transitions field from migration_plan
- Added _active_transitions member to load_balancer class
- Updated make_plan() to reset counter at the start
- Log messages now reference _active_transitions directly
Co-authored-by: tgrabiec <283695+tgrabiec@users.noreply.github.com>
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