IsInitialBlockDownload: usually avoid locking

Optimistically test the latch bool before taking the lock.
For all IsInitialBlockDownload calls after the first to return false,
this avoids the need to lock cs_main.

Coming from btc@f0fdda0181e1b05b66541bf235c6702c41664170

src/main.cpp

@@ -1530,17 +1530,22 @@ int64_t GetMasternodePayment()
 
 bool IsInitialBlockDownload()
 {
+    // Once this function has returned false, it must remain false.
+    static std::atomic<bool> latchToFalse{false};
+    // Optimization: pre-test latch before taking the lock.
+    if (latchToFalse.load(std::memory_order_relaxed))
+        return false;
+
     LOCK(cs_main);
+    if (latchToFalse.load(std::memory_order_relaxed))
+         return false;
     const int chainHeight = chainActive.Height();
     if (fImporting || fReindex || fVerifyingBlocks || chainHeight < Checkpoints::GetTotalBlocksEstimate())
         return true;
-    static bool lockIBDState = false;
-    if (lockIBDState)
-        return false;
     bool state = (chainHeight < pindexBestHeader->nHeight - 24 * 6 ||
             pindexBestHeader->GetBlockTime() < GetTime() - nMaxTipAge);
     if (!state)
-        lockIBDState = true;
+        latchToFalse.store(true, std::memory_order_relaxed);
     return state;
 }