Merge #9053: IBD using chainwork instead of height and not using header timestamps

e141beb IsInitialBlockDownload no longer uses header-only timestamps. (Gregory Maxwell)
2082b55 Remove GetTotalBlocksEstimate and checkpoint tests that test nothing. (Gregory Maxwell)
fd46136 IBD check uses minimumchain work instead of checkpoints. (Gregory Maxwell)

src/main.cpp

@@ -1746,13 +1746,14 @@ bool IsInitialBlockDownload()
         return false;
     if (fImporting || fReindex)
         return true;
-    if (fCheckpointsEnabled && chainActive.Height() < Checkpoints::GetTotalBlocksEstimate(chainParams.Checkpoints()))
+    if (chainActive.Tip() == NULL)
         return true;
-    bool state = (chainActive.Height() < pindexBestHeader->nHeight - 24 * 6 ||
-            std::max(chainActive.Tip()->GetBlockTime(), pindexBestHeader->GetBlockTime()) < GetTime() - nMaxTipAge);
-    if (!state)
-        latchToFalse.store(true, std::memory_order_relaxed);
-    return state;
+    if (chainActive.Tip()->nChainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
+        return true;
+    if (chainActive.Tip()->GetBlockTime() < (GetTime() - nMaxTipAge))
+        return true;
+    latchToFalse.store(true, std::memory_order_relaxed);
+    return false;
 }
 
 bool fLargeWorkForkFound = false;
@@ -1780,7 +1781,7 @@ void CheckForkWarningConditions()
 {
     AssertLockHeld(cs_main);
     // Before we get past initial download, we cannot reliably alert about forks
-    // (we assume we don't get stuck on a fork before the last checkpoint)
+    // (we assume we don't get stuck on a fork before finishing our initial sync)
     if (IsInitialBlockDownload())
         return;