[#38] service/object: Implement simplified object Head service

Implement Head service w/o linking object processing and restoration from
split-chain.

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

pkg/services/object/head/util.go

@@ -0,0 +1,27 @@
+package headsvc
+
+import (
+	"context"
+	"sync"
+
+	"github.com/nspcc-dev/neofs-node/pkg/core/object"
+	"github.com/nspcc-dev/neofs-node/pkg/services/object_manager/placement"
+)
+
+type onceHeaderWriter struct {
+	once *sync.Once
+
+	traverser *placement.Traverser
+
+	resp *Response
+
+	cancel context.CancelFunc
+}
+
+func (w *onceHeaderWriter) write(hdr *object.Object) {
+	w.once.Do(func() {
+		w.resp.hdr = hdr
+		w.traverser.SubmitSuccess()
+		w.cancel()
+	})
+}