[#378] shard: Collect expired non-tombstone objects in GC every epoch

Add new epoch event handler to GC that finds all expired non-tombstone
objects and marks them to be removed.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/shard/control.go

@@ -40,9 +40,16 @@ func (s *Shard) Init() error {
 	}
 
 	gc := &gc{
-		gcCfg:         s.gcCfg,
-		remover:       s.removeGarbage,
-		mEventHandler: map[eventType]*eventHandlers{},
+		gcCfg:   s.gcCfg,
+		remover: s.removeGarbage,
+		mEventHandler: map[eventType]*eventHandlers{
+			eventNewEpoch: {
+				cancelFunc: func() {},
+				handlers: []eventHandler{
+					s.collectExpiredObjects,
+				},
+			},
+		},
 	}
 
 	gc.init()