[#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/options.go

@@ -44,6 +44,8 @@ type options struct {
 	disableBackgroundFlush bool
 	// pageSize is bbolt's page size config value
 	pageSize int
+	// flushSizeLimit is total size of flushing objects.
+	flushSizeLimit uint64
 }
 
 // WithLogger sets logger.
@@ -169,3 +171,10 @@ func WithPageSize(s int) Option {
 		o.pageSize = s
 	}
 }
+
+// WithFlushSizeLimit sets flush size limit.
+func WithFlushSizeLimit(v uint64) Option {
+	return func(o *options) {
+		o.flushSizeLimit = v
+	}
+}