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
This is a backport of Core PR19132
Depends on D9254