net: dedup and RAII-fy the creation of a copy of CConnman::vNodes

The following pattern was duplicated in CConnman:

```cpp
lock
create a copy of vNodes, add a reference to each one
unlock
... use the copy ...
lock
release each node from the copy
unlock
```

Put that code in a RAII helper that reduces it to:

```cpp
create snapshot "snap"
... use the copy ...
// release happens when "snap" goes out of scope
```

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