[#1367] writecache: Add background flushing objects limiter

To limit memory usage by background flush.

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

pkg/local_object_storage/writecache/limiter_test.go

@@ -0,0 +1,27 @@
+package writecache
+
+import (
+	"sync/atomic"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sync/errgroup"
+)
+
+func TestLimiter(t *testing.T) {
+	var maxSize uint64 = 10
+	var single uint64 = 3
+	l := newFlushLimiter(uint64(maxSize))
+	var currSize atomic.Int64
+	var eg errgroup.Group
+	for i := 0; i < 10_000; i++ {
+		eg.Go(func() error {
+			defer l.release(single)
+			defer currSize.Add(-1)
+			l.acquire(single)
+			require.True(t, currSize.Add(1) <= 3)
+			return nil
+		})
+	}
+	require.NoError(t, eg.Wait())
+}