[#109] services: Implement Replicator service

Implement Replicator service that performs background work to replicate
local object to remote nodes in the container. Replicator is going to be
used by Policer.

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

pkg/services/replicator/task.go

@@ -0,0 +1,53 @@
+package replicator
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg/netmap"
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+)
+
+// Task represents group of Replicator task parameters.
+type Task struct {
+	quantity uint32
+
+	addr *object.Address
+
+	nodes netmap.Nodes
+}
+
+// AddTask pushes replication task to Replicator queue.
+//
+// If task queue is full, log message is written.
+func (p *Replicator) AddTask(t *Task) {
+	select {
+	case p.ch <- t:
+	default:
+		p.log.Warn("task queue is full")
+	}
+}
+
+// WithCopiesNumber sets number of copies to replicate.
+func (t *Task) WithCopiesNumber(v uint32) *Task {
+	if t != nil {
+		t.quantity = v
+	}
+
+	return t
+}
+
+// WithObjectAddress sets address of local object.
+func (t *Task) WithObjectAddress(v *object.Address) *Task {
+	if t != nil {
+		t.addr = v
+	}
+
+	return t
+}
+
+// WithNodes sets list of potential object holders.
+func (t *Task) WithNodes(v netmap.Nodes) *Task {
+	if t != nil {
+		t.nodes = v
+	}
+
+	return t
+}