It is not possible to have a node in `CConnman::vNodesDisconnected` and
its reference count to be incremented - all `CNode::AddRef()` are done
either before the node is added to `CConnman::vNodes` or while holding
`CConnman::cs_vNodes` and the object being in `CConnman::vNodes`.
So, the object being in `CConnman::vNodesDisconnected` and its reference
count being zero means that it is not and will not start to be used by
other threads.
So, the lock of `CNode::cs_vSend` in `CConnman::DisconnectNodes()` will
always succeed and is not necessary.
Indeed all locks of `CNode::cs_vSend` are done either when the reference
count is >0 or under the protection of `CConnman::cs_vNodes` and the
node being in `CConnman::vNodes`.
This is a backport of core#21750