[#661] blobovniczatree: Make Rebuild concurrent

Different DBs can be rebuild concurrently.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/engine/rebuild_limiter.go

@@ -0,0 +1,26 @@
+package engine
+
+import "context"
+
+type rebuildLimiter struct {
+	semaphore chan struct{}
+}
+
+func newRebuildLimiter(workersCount uint32) *rebuildLimiter {
+	return &rebuildLimiter{
+		semaphore: make(chan struct{}, workersCount),
+	}
+}
+
+func (l *rebuildLimiter) AcquireWorkSlot(ctx context.Context) error {
+	select {
+	case l.semaphore <- struct{}{}:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}
+
+func (l *rebuildLimiter) ReleaseWorkSlot() {
+	<-l.semaphore
+}