refactor(ecstore): migrate the bucket bandwidth monitor into InstanceContext (Phase 5 Slice 10)
Phase 5 Slice 10 (backlog#939): move the bucket bandwidth monitor out of the
process static into the per-instance InstanceContext.
- InstanceContext gains `bucket_monitor: OnceLock<Arc<Monitor>>` with
`init_bucket_monitor(num_nodes)` (set-once; returns false if already set) and
`bucket_monitor() -> Option<..>`.
- global.rs `init_global_bucket_monitor` / `get_global_bucket_monitor` keep
their signatures and route through the current instance's context; the static
is removed. The ignore-and-warn-on-reinit behavior is preserved (the warn
stays in global.rs).
Tests: the monitor is None until initialized, set-once (second init is a no-op),
and independent across instances.
Verification: cargo test -p rustfs-ecstore (21 instance-context tests green),
cargo clippy -p rustfs-ecstore --all-targets (clean), make pre-commit (pass).
Refs: backlog#939 (Phase 5, Slice 10).
