refactor: consolidate 3 separate locks into one block

The main lock needs to be taken 3 separate times.
It simplifies readability to take it once, and is more efficient.

src/validation.cpp

@@ -3560,10 +3560,12 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
         } // release cs_main
         // When we reach this point, we switched to a new tip (stored in pindexNewTip).
 
+        bool disabled{false};
+        {
+            LOCK(m_chainman.GetMutex());
             if (exited_ibd) {
                 // If a background chainstate is in use, we may need to rebalance our
                 // allocation of caches once a chainstate exits initial block download.
-            LOCK(::cs_main);
                 m_chainman.MaybeRebalanceCaches();
             }
 
@@ -3572,7 +3574,10 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
                 return false;
             }
 
-        if (WITH_LOCK(::cs_main, return m_disabled)) {
+            disabled = m_disabled;
+        }
+
+        if (disabled) {
             // Background chainstate has reached the snapshot base block, so exit.
 
             // Restart indexes to resume indexing for all blocks unique to the snapshot