[#329] node: Make evacuate async

Now it's possible to run evacuate shard in async.
Also only one evacuate process can be in progress.

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

pkg/local_object_storage/engine/engine.go

@@ -35,6 +35,7 @@ type StorageEngine struct {
 
 		err error
 	}
+	evacuateLimiter *evacuationLimiter
 }
 
 type shardWrapper struct {
@@ -230,6 +231,9 @@ func New(opts ...Option) *StorageEngine {
 		shardPools: make(map[string]util.WorkerPool),
 		closeCh:    make(chan struct{}),
 		setModeCh:  make(chan setModeRequest),
+		evacuateLimiter: &evacuationLimiter{
+			guard: &sync.RWMutex{},
+		},
 	}
 }