[#378] shard: Initialize GC processes on Init

`Shard.Init` method creates a new GC instance from shard configuration and
starts GC's workers through `init` call. In initial implementation GC
routines are indefinite and can be killed only with by application shutdown.

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

pkg/local_object_storage/shard/control.go

@@ -39,6 +39,14 @@ func (s *Shard) Init() error {
 		}
 	}
 
+	gc := &gc{
+		gcCfg:         s.gcCfg,
+		remover:       s.removeGarbage,
+		mEventHandler: map[eventType]*eventHandlers{},
+	}
+
+	gc.init()
+
 	return nil
 }