devmapper: async remove device using Cleanup

Fix: #3923
Signed-off-by: Eric Ren <renzhen@linux.alibaba.com>

snapshots/devmapper/config.go

@@ -40,6 +40,9 @@ type Config struct {
 	// Defines how much space to allocate when creating base image for container
 	BaseImageSize      string `toml:"base_image_size"`
 	BaseImageSizeBytes uint64 `toml:"-"`
+
+	// Flag to async remove device using Cleanup() callback in snapshots GC
+	AsyncRemove bool `toml:"async_remove"`
 }
 
 // LoadConfig reads devmapper configuration file from disk in TOML format