[#108] services: Implement Policer service

Implement Policer service that performs background work to check compliance
with the placement policy for local objects in the container. In the initial
implementation, the selection of the working queue of objects is
simplified, and there is no transfer of the result to the replicator.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/policer/queue.go

@@ -0,0 +1,28 @@
+package policer
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/localstore"
+)
+
+type jobQueue struct {
+	localStorage *localstore.Storage
+}
+
+func (q *jobQueue) Select(limit int) ([]*object.Address, error) {
+	// TODO: optimize the logic for selecting objects
+	// We can prioritize objects for migration, newly arrived objects, etc.
+	// It is recommended to make changes after updating the metabase
+
+	res := make([]*object.Address, 0, limit)
+
+	if err := q.localStorage.Iterate(nil, func(meta *localstore.ObjectMeta) bool {
+		res = append(res, meta.Head().Address())
+
+		return len(res) >= limit
+	}); err != nil {
+		return nil, err
+	}
+
+	return res, nil
+}