[#92] Refactor policer and add some unit tests

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/services/policer/nodecache.go

@@ -0,0 +1,57 @@
+package policer
+
+import "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/netmap"
+
+type nodeProcessStatus int8
+
+const (
+	nodeNotProcessed nodeProcessStatus = iota
+	nodeDoesNotHoldObject
+	nodeHoldsObject
+)
+
+func (st nodeProcessStatus) Processed() bool {
+	return st != nodeNotProcessed
+}
+
+// nodeCache tracks Policer's check progress.
+type nodeCache map[uint64]bool
+
+func newNodeCache() nodeCache {
+	return make(map[uint64]bool)
+}
+
+func (n nodeCache) set(node netmap.NodeInfo, val bool) {
+	n[node.Hash()] = val
+}
+
+// submits storage node as a candidate to store the object replica in case of
+// shortage.
+func (n nodeCache) submitReplicaCandidate(node netmap.NodeInfo) {
+	n.set(node, false)
+}
+
+// submits storage node as a current object replica holder.
+func (n nodeCache) submitReplicaHolder(node netmap.NodeInfo) {
+	n.set(node, true)
+}
+
+// processStatus returns current processing status of the storage node.
+func (n nodeCache) processStatus(node netmap.NodeInfo) nodeProcessStatus {
+	switch val, ok := n[node.Hash()]; {
+	case !ok:
+		return nodeNotProcessed
+	case val:
+		return nodeHoldsObject
+	default:
+		return nodeDoesNotHoldObject
+	}
+}
+
+// SubmitSuccessfulReplication marks given storage node as a current object
+// replica holder.
+//
+// SubmitSuccessfulReplication implements replicator.TaskResult.
+func (n nodeCache) SubmitSuccessfulReplication(node netmap.NodeInfo) {
+	n.submitReplicaHolder(node)
+}