fix: lock ::cs_main before accessing `ChainstateManager::m_best_hea…

…der`

Avoid TSan-reported data race

```
WARNING: ThreadSanitizer: data race (pid=360336)
  Write of size 8 at 0x7b5000002db8 by thread T15 (mutexes: write M0):
    #0 BlockManager::AddToBlockIndex(CBlockHeader const&, uint256 const&, CBlockIndex*&, BlockStatus) /src/dash/src/node/blockstorage.cpp:117:25 (dashd+0x44df7a) (BuildId: e27186c7ba08e897d376eb5779db9809baf7ad37)
    #1 ChainstateManager::AcceptBlockHeader(CBlockHeader const&, BlockValidationState&, CChainParams const&, CBlockIndex**) /src/dash/src/validation.cpp:3758:36 (dashd+0x83e45d) (BuildId: e27186c7ba08e897d376eb5779db9809baf7ad37)
    #2 CChainState::AcceptBlock(std::shared_ptr<CBlock const> const&, BlockValidationState&, CBlockIndex**, bool, FlatFilePos const*, bool*) /src/dash/src/validation.cpp:3812:37 (dashd+0x842848) (BuildId: e27186c7ba08e897d376eb5779db9809baf7ad37)
    [...]
  Previous read of size 8 at 0x7b5000002db8 by thread T12:
    #0 CDSNotificationInterface::UpdatedBlockTip(CBlockIndex const*, CBlockIndex const*, bool) /src/dash/src/dsnotificationinterface.cpp:82:42 (dashd+0x91e87e) (BuildId: e27186c7ba08e897d376eb5779db9809baf7ad37)
    #1 operator() /src/dash/src/validationinterface.cpp:199:79 (dashd+0x889f1e) (BuildId: e27186c7ba08e897d376eb5779db9809baf7ad37)
    #2 Iterate<(lambda at validationinterface.cpp:199:30)> /src/dash/src/validationinterface.cpp:88:17 (dashd+0x889f1e)
    [...]
[...]
SUMMARY: ThreadSanitizer: data race [...]
```

src/dsnotificationinterface.cpp

@@ -79,7 +79,7 @@ void CDSNotificationInterface::UpdatedBlockTip(const CBlockIndex *pindexNew, con
  if (pindexNew == pindexFork) // blocks were disconnected without any new ones
  return;
 
- m_mn_sync.UpdatedBlockTip(m_chainman.m_best_header, pindexNew, fInitialDownload);
+ m_mn_sync.UpdatedBlockTip(WITH_LOCK(::cs_main, return m_chainman.m_best_header), pindexNew, fInitialDownload);
 
  // Update global DIP0001 activation status
  fDIP0001ActiveAtTip = pindexNew->nHeight >= Params().GetConsensus().DIP0001Height;