[#661] blobovniczatree: Make Rebuild concurrent

Different DBs can be rebuild concurrently.

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

pkg/local_object_storage/shard/rebuild_limiter.go

@@ -0,0 +1,13 @@
+package shard
+
+import "context"
+
+type RebuildWorkerLimiter interface {
+	AcquireWorkSlot(ctx context.Context) error
+	ReleaseWorkSlot()
+}
+
+type noopRebuildLimiter struct{}
+
+func (l *noopRebuildLimiter) AcquireWorkSlot(context.Context) error { return nil }
+func (l *noopRebuildLimiter) ReleaseWorkSlot()                      {}