qt: lock cs_main, m_cached_tip_mutex in that order

Always lock the mutexes `cs_main` and `m_cached_tip_mutex` in
the same order: `cs_main`, `m_cached_tip_mutex`. Otherwise we may end up
in a deadlock.

`ClientModel::m_cached_tip_blocks` is protected by
`ClientModel::m_cached_tip_mutex`. There are two access paths that
lock the two mutexes in opposite order:

```
validation.cpp:2868 CChainState::ActivateBestChain(): lock cs_main
validation.cpp:2916 CChainState::ActivateBestChain(): call uiInterface.NotifyBlockTip()
ui_interface.cpp:52 CClientUIInterface::NotifyBlockTip(): go deep in boost
...
qt/clientmodel.cpp:255 BlockTipChanged(): lock m_cached_tip_mutex
```

and

```
qt/clientmodel.cpp:119 ClientModel::getBestBlockHash(): lock m_cached_tip_mutex
qt/clientmodel.cpp:121 ClientModel::getBestBlockHash(): call m_node.getBestBlockHash()
interfaces/node.cpp:200 NodeImpl::getBestBlockHash(): lock cs_main
```

From `debug.log`:

```
POTENTIAL DEADLOCK DETECTED
Previous lock order was:
 m_cs_chainstate validation.cpp:2851
 (1) cs_main validation.cpp:2868
 ::mempool.cs validation.cpp:2868
 (2) clientmodel->m_cached_tip_mutex qt/clientmodel.cpp:255
Current lock order is:
 (2) m_cached_tip_mutex qt/clientmodel.cpp:119
 (1) ::cs_main interfaces/node.cpp:200
```

The possible deadlock was introduced in
https://github.com/bitcoin/bitcoin/pull/17993

src/qt/clientmodel.cpp

@@ -116,9 +116,23 @@ int ClientModel::getNumBlocks() const
 
 uint256 ClientModel::getBestBlockHash()
 {
+    uint256 tip{WITH_LOCK(m_cached_tip_mutex, return m_cached_tip_blocks)};
+
+    if (!tip.IsNull()) {
+        return tip;
+    }
+
+    // Lock order must be: first `cs_main`, then `m_cached_tip_mutex`.
+    // The following will lock `cs_main` (and release it), so we must not
+    // own `m_cached_tip_mutex` here.
+    tip = m_node.getBestBlockHash();
+
     LOCK(m_cached_tip_mutex);
+    // We checked that `m_cached_tip_blocks` is not null above, but then we
+    // released the mutex `m_cached_tip_mutex`, so it could have changed in the
+    // meantime. Thus, check again.
     if (m_cached_tip_blocks.IsNull()) {
-        m_cached_tip_blocks = m_node.getBestBlockHash();
+        m_cached_tip_blocks = tip;
     }
     return m_cached_tip_blocks;
 }