bug redis: disconnect slots on stop in standalone topology holder

`ClusterTopologyHolder::Stop()` was not disconnecting the slots, resulting in new events apearing after the `Stop()`:

```
/userver/redis/src/storages/redis/impl/cluster_sentinel_impl.cpp:886:13: runtime error: member access within address 0x51100001e040 which does not point to an object of type 'userver_ns::storages::redis::impl::ClusterSentinelImpl'
1: 0x51100001e040: note: object has invalid vptr
1:  04 00 00 00  fc 42 00 00 03 00 00 00  d0 7d 01 00 30 51 00 00  80 39 06 00 60 51 00 00  00 00 00 00
1:               ^~~~~~~~~~~~~~~~~~~~~~~
1:               invalid vptr
```

Tests: протестировано CI
commit_hash:339ccdcc1e6deeb0e2ac2f733d33c9f82edfdf21

Author: apolukhin

redis/src/storages/redis/impl/standalone_topology_holder.cpp

@@ -38,6 +38,9 @@ void StandaloneTopologyHolder::Start() {
 }
 
 void StandaloneTopologyHolder::Stop() {
+    signal_node_state_change_.disconnect_all_slots();
+    signal_topology_changed_.disconnect_all_slots();
+
     node_.Cleanup();
     topology_.Cleanup();
 }