Merge #1728: IsInitialBlockDownload: usually avoid locking

9e378e4 IsInitialBlockDownload: usually avoid locking (furszy)

Pull request description:

  Straightforward, coming from [btc@8007](bitcoin#8007)

ACKs for top commit:
  random-zebra:
    Nice! utACK 9e378e4
  Fuzzbawls:
    utACK 9e378e4

Tree-SHA512: 3f5ebfbafa64ca7ff5397405c87f0030d81a4342486e420c7f3bf03daef08947afb22aed9d20bff02b4376a64e02902b9882c637e7a94ec235c4a5d4e1280391

src/main.cpp

@@ -1554,17 +1554,22 @@ 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;
 }